GB2434893A

GB2434893A - Method for storing recovery data at a remote site as chucks of data covering known time periods, with synchronous data sent to a local minimal storage

Info

Publication number: GB2434893A
Application number: GB0702668A
Authority: GB
Inventors: Douglas E Lecrone; Gary H Cox; Benjamin W Yoder; David Meiri; Mark J Halstead; Brett A Quinn
Original assignee: EMC Corp
Current assignee: EMC Corp
Priority date: 2004-09-30
Filing date: 2005-09-30
Publication date: 2007-08-08
Anticipated expiration: 2025-09-30
Also published as: GB2434894B; GB2434894A8; DE102005036291B9; GB2434893A8; GB2434893B; DE102005036291A1; GB0702668D0; DE102005036291B4; GB0702669D0; GB2434894A; GB2434894B8; GB2434893B8; JP2006107450A

Abstract

Disclosed is a method for storing recovery data at a remote destination or location. The data is sent in chucks, each of which represents data covering a known time period, with a subsequent chuck covering the immediately following time period. Synchronous data, corresponding to the data chunks, is also sent to a local minimal storage location such that the local store contains data that is not stored on the remote store and the remote store contains data that is not stored locally. An indicator that a new chuck of data is being sent is provided to the storage locations. The data stored in the remote store may be older than the data stored locally. The data stored at the local store may be stored as a linked list of elements, which are sorted according to the location of the data source or according to the order of receipt. A local cycle number may be provided at the local store and be incremented when the new data chuck indicator is received.

Description

TRIANGULAR ASYNCHRONOUS REPLICATION

This application relates to computer storage devices, and more particularly to the field of transferring data between storage devices.

Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Patent No 5,206,939 to Yanai et al., U.S. Patent No. 5,778,394 to Galtzur Ct al., U.S. Patent No. 5,845,147 to Vish]itzky et al., and U.S. Patent No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.

In some instances, it may be desirable to copy data from one storage device to another. For example, if a host writes data to a first storage device, it may be desirable to copy that data to a second storage device provided in a different location so that if a disaster occurs that renders the first storage device inoperable, the host (or another host) may resume operation using the data of the second storage device. Such a capability is provided, for example, by the Remote Data Facility (RDF) product provided by EMC Corporation of Hopkinton, Massachusetts. With RDF, a first storage device, denoted the "primary storage device" (or "Ri ") is coupled to the host. One or more other storage devices, called "secondary storage devices" (or "R2") receive côpies of the data that is written to the primary storage device by the host. The host interact directly with the primary storage device, but any data changes made to the primary stoage device are automatically provided to the one or more secondary storage devices psng RDF.

The primary and secondary storage devices may be connected by a data link, such as an ESCON link, a Fibre Channel link, andlor a Gigabit Ethernet link. The RDF functionality may be facilitated with an RDF adapter (RA) provided at each of the storage devices.

RDF allows synchronous data transfer where, after data written frofl a host to a primary storage device is transferred from the primary storage device to a secondary storage device using RDF, receipt is acknowledged by the secondary storage device to the primary storage device which then provides a write acknowledge back to the host.

Thus, in synchronous mode, the host does not receive a write acknowledge from the primary storage device until the RDF transfer to the secondary storage device has been completed and acknowledged by the secondary storage device.

A drawback to the synchronous RDF system is that the latency of each of the write operations is increased by waiting for the acknowledgement of the RDF transfer.

This problem is worse when there is a long distance between the primary storage device and the secondary storage device; because of transmission delays, the time delay required for making the RDF transfer and then waiting for an acknowledgement back after the transfer is complete may be unacceptable.

It is also possible to use RDF in an a semi-synchronous mode, in which case the data is written from the host to the primary storage device which acknowledges the write immediately and then, at the same time, begins the process of transferring the data to the secondary storage device. Thus, for a single transfer of data, this scheme overcomes some of the disadvantages of using RDF in the synchronous mode.

However, for data integrity purposes, the semi-synchronous transfer mode does not allow the primary storage device to transfer data to the secondary storage device until a previous transfer is acknowledged by the secondary storage device. Thus, the bottlenecks associated with using RDF in the synchronous mode are simply delayed by one iteration because transfer of a second amount of data cannot occur until transfer of previous data has been acknowledged by the secondary storage device.

Another possibility is to have the host write data to the primary storage device in asynchronous mode and have the primary storage device copy data to the secondary storage device in the background. The background copy involves cycling through each of the fracks of the primary storage device sequentially and, when it is determined that a particular block has been modified since the last time that block was copied, the block is transferred from the primary storage device to the secondary storage device.

Although this mechanism may attenuate the latency problem associated with synchronous and semi-synchronous data transfer modes, a difficulty still exists because there can not be a guarantee of data consistency between the primary and secondary storage devices. If there are problems, such as a failure of the primary system, the secondary system may end up with out-of-order changes that make the data unusable.

A proposed solution to this problem is the Symmetrix Automated Replication (SAR) process, which is described in pending U.S. patent applications 10/224,918 and 10/225,021, both of which were filed on August 21, 2002. The SAR uses devices (BCV's) that can minor standard logical devices. A BCV device can also be split from its standard logical device after being mirrored and can be resynced (i.e., reestablished as a mirror) to the standard logical devices after being split. In addition, a BCV can be remotely mirrored using RDF, in which case the BCV may propagate data changes made thereto (while the BCV is acting as a mirror) to the BCV remote minor when the BCV is split from the corresponding standard logical device.

However, using the SAR process requires the significant overhead of continuously splitting and resyncing the BCV's. The SAR process also uses host control and management, which relies on the controlling host being operational. in addition, the cycle time for a practical implementation of a SAR process is on the order of twenty to thirty minutes, and thus the amount of data that may be lost when an RDF link and/or primary device fails could be twenty to thirty minutes worth of data.

Thus, it would be desirable to have an RDF system that exhibits some of the beneficial qualities of each of the different techniques discussed above while reducing the drawbacks. Such a system would exhibit low latency for each host write regardless of the distance between the primary device and the secondary device and would provide consistency (recoverability) of the secondary device in case of failure.

It would also be desirable to be able to combine the benefits obtained from synchronous RDF transfers and asynchronous RDF transfers so that up-to-date backup S data may be provided on a JO remote device that is relatively close (geographically) to a source device while, at the same time, backup data may also be provided to a backup device that is relatively far from the source device. It would also be desirable if such a system provided for appropriate data recovery among the backup devices.

According to the present invention, storing recovery data includes providing chunks of data to a remote destination, where each chunk of data represents data written before a first time and aftei i second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks, providing synchronous data corresponding to the chunks of data to a minimal storage local destination, where the minimal storage local destination contains data that is not stored on the remote destination and where the remote destination contains data that is not stored on the minimal storage local destination, and providing an indicator when a new chunk of data is being provided. The data that is contained on the remote destination but not on the minimal storage local destination may be older than the data that is contained on the minimal storage local destination but not on the remote destination. The synchronous data received by the minimal storage local destination may be stored in a linked list of elements. The elements may be sorted according to a source storage device location of the data. The elements may be sorted according to an order of receipt of the data at the minimal storage local destination. Each of the elements may include an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements. Storing recovery data may also include providing a local cycle number at the minimal storage remote destination and, in response to receiving the indicator that a new chunk of data is being provided, incrementing the local indicator. The synchronous data received by the minimal storage local destination may be stored in a linked list of elements. Each of the elements may include an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements and the identifier may correspond to the local cycle number.

According further to the present invention, computer software, provided in a computer readable medium, that stores recovery data includes executable code that provides chunks of data to a remote destination where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks, executable code that provides synchronous data corresponding to the chunks of data to a minimal storage local destination, where the minimal storage local destination contains data that is not stored on the remote destination and where the remote destination contains data that is not stored on the minimal storage local destination and executable code that provides an indicator when a new chunk of data is being provided. The data that is contained on the remote destination but not on the minimal storage local destination may be older than the data that is contained on the minimal storage local destination but not on the remote destination. The synchronous data received by the minimal storage local destination may be stored in a linked list of elements. The elements may be sorted according to a source storage device location of the data. The elements may be sorted according to an order of receipt of the data at the minimal storage local destination. Each of the elements may include an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements. The computer software may also include executable code that provides a local cycle number at the minimal storage remote destination and executable code that increments the local indicator in response to receiving the indicator that a new chunk of data is being provided. The synchronous data received by the minimal storage local destination may be stored in a linked list of elements. Each of the elements may include an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements and wherein the identifier corresponds to the local cycle number.

According further to the present invention, a system for storing recovery data includes a source group, a remote destination coupled to the source group to receive

S

therefrom chunks of data, where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks and a minimal storage local destination coupled to the source group to receive synchronous data therefrom, where. the minimal storage local destination contains data that is not stored on the remote destination and the remote destination contains data that is not stored on the minimal storage local destination and where the source group provides an indicator when a new chunk of data is being provided. The data that is contained on the remote destination but not on the minimal storage local destination may be older than the data that is contained on the minimal storage local destination but not on the remote destination.

According further to the present invention, storing recovery data includes providing chunks of data to a remote destination, where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks, providing synchronous data to a local destination, and providing an indicator to the local destination in connection with creation of a new chunk of data for storage at the remote destination. The local destination may maintain a plurality of maps, where each of the maps associates synchronous data being provided thereto with a specific chunk of data. In response to receiving an indicator in connection with creation of a new chunk of data, the local destination may point to a new map. There may be two maps or more than two maps. In response to the local destination failing to acknowledge synchronous data provided thereto, the remote destination may maintain a map of data written thereto. Storing recovery data may include, in response to the local destination failing to acknowledge synchronous data provided thereto, the remote destination maintaining a count of a number of times a new chunk of data is created.

According further to the present invention, recovering data stored on a local destination and a remote destination includes determining which of the local destination and the remote has the most up-to-date data, copying data from the remote destination to the local destination if the remote destination has the most up-to-date data, and copying data from the local destination to the remote destination if the local destination has the most up-to-date data. Determining which of the local destination and the remote destination has the most up-to-date data may include examining a token provided by the remote destination that indicates an amount of new data that is stored on the remote destination that is not stored on the local destination. The token may indicate a number of new chunks of data provided to the remote storage device following failure of the local storage device to acknowledge data provided thereto, where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks. Copying data from the remote destination to the local destination may include copying data indicated by a map corresponding to data written to the remote destination that is not acknowledged as being written to the local destination. Copying data from the local destination to the remote destination may include copying data indicated by a plurality of maps corresponding to data written to the local destination that is not acknowledged as being written to the remote destination. There may be two or more than two maps.

According further to the present invention, software that stores recovery data includes executable code that provides chunks of data to a remote destination, where each chunk of data represents data written before a first time and ftèf a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks, executable code that provides synchronous data to a local destination, and executable code that provides an indicator to the local destination in connection with creation of a new chunk of data for storage at the remote destination. The local destination may include executable code that maintains a plurality of maps, where each of the maps associates synchronous data being provided thereto with a specific chunk of data. In response to receiving an indicator in connection with creation of a new chunk of data, the local destination may point to a new map. There may be two maps or more than two maps. The software may include executable code at the remote destination that, in response to the local destination failing to acknowledge synchronous data provided thereto, maintains a map of data provided thereto. The software may include executable code at the remote destination that, in response to the local destination failing to acknowledge synchronous data provided thereto, maintains a count of a number of times a new chunk of data is created.

According further to the present invention, software that recovers data stored on a local destination and a remote destination includes executable code that determines which of the local destination and the remote has the most up-to-date data, executable code that copies data from the remote destination to the local destination if the remote destination has the most up-to-date data, and executable code that copies data from the local destination to the remote destination if the local destination has the most up-to-date data. Executable code that determines which of the local destination and the remote destination has the most up-to-date data may include executable code that examines a token provided by the remote destination that indicates an amount of new data that is stored on the remote destination that is not stored on the local destination.

The token may indicate a number of new chunks of data provided to the remote storage device following failure of the local storage device to acknowledge data provided thereto, where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks. Executable code that copies data from the remote destination to the local destination may include executable code that copies data indicated by a map corresponding to data written to the remote destination that is not acknowledged as being written to the local destination.

Executable code that copies data from the local destination to the remote destination may include executable code that copies data indicated by a plurality of maps corresponding to data written to the local destination that is not acknowledged as being written to the remote destination. There may be two maps or more than two maps.

According further to the present invention, a system for storing recovery data includes a source group, a remote destination coupled to the source group to receive therefrom chunks of data, where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks, and a local destination coupled to the source group to receive synchronous data therefrom, where the source group provides an indicator to the local destination in connection with creation of a new chunk of data for storage at the remote destination.

According further to the present invention, a system for recovering data includes a source group that generates data, a local destination, coupled to the source group to receive synchronous data therefrom, a remote destination, coupled to the source group to receive asynchronous data therefrom, means for determining which of the local destination and the remote has the most up-to-date data, means for copying data from the remote destination to the local destination if the remote destination has the most up-to-date data, and mean for copying data from the local destination to the remote destination if the local destination has the most up-to-date data.

According further to the present invention, storing recovery data includes host processor writing data to a local storage device, the host processor causing the local storage device to accumulate chunks of data corresponding to writes by the host processor, where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks of data corresponds to a first time for a subsequent one of the particular chunks of data, transmitting the chunks of data from the local storage device to a remote destination, providing synchronous data from the local storage device to a local destination; and, the host processor causing an indicator to be provided to the local destination in connection with creation of a new chunk of data for storage at the remote destination. The local destination may maintain a plurality of maps, where each of the maps associates synchronous data being provided thereto with a specific chunk of data. In response to receiving an indicator in connection with creation of a new chunk of data, the local destination may point to a new map. There may be two maps or more than two maps.

In response to the local destination failing to acknowledge synchronous data provided thereto, the remote destination may maintain a map of data written thereto. Storing recovery data may include, in response to the local destination failing to acknowledge synchronous data provided thereto, the remote destination maintaining a count of a number of times a new chunk of data is created. The host processor may write data to more than one local storage device that synchronously transfer data to more than one local destination.

According further to the present invention, recovering data stored on a local destination and a remote destination includes causing a host to locally access one of: the local destination and the remote destination, the host determining which of the local destination and the remote destination has the most up-to-date data, the host causing data to be copied from the remote destination to the local destination if the remote destination has the most up-to-date data, and, the host causing data to be copied from the local destination to the remote destination if the local destination has the most up-to-date data. The host determining which of the local destination and the remote destination has the most up-to-date data may include the host examining a token provided by the remote destination that indicates an amount of new data that is stored on the remote destination that is not stored on the local destination. The token may indicate a number of new chunks of data provided to the remote storage device following failure of the local storage device to acknowledge data provided thereto, where each chunk of data represents data written before a first time and after a second time and where the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks. The host copying data from the remote destination to the local destination may include the host copying data indicated by a map corresponding to data written to the remote destination that is not acknowledged as being written to the local destination. The host copying data from the local destination to the remote destination may include the host copying data indicated by a plurality of maps corresponding to data written to the local destination that is not acknowledged as being written to the remote destination. The number of maps may be two or may be more than two.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a schematic diagram showing a host, a local storage device, and a remote data storage device used in connection with the system described herein.

Figure 2 is a schematic diagram showing a flow of data between a host, a local storage device, and a remote data storage device used in connection with the system described herein.

Figure 3 is a schematic diagram illustrating items for constructing and manipulating chunks of data on a local storage device according to the system described herein.

Figure 4 is a diagram illustrating a data structure for a slot used in connection with the system described herein.

Figure 5 is a flow chart illustrating operation of a host adaptor (HA) in response to a write by a host according to the system described herein.

Figure 6 is a flow chart illustrating transferring data from a local storage device to a remote storage device according to the system described herein.

Figure 7 is a schematic diagram illustrating items for constructing and manipulating chunks of data on a remote storage device according to the system described herein.

Figure 8 is a flow chart illustrating steps performed by a remote storage device in connection with receiving I a commit indicator from a local storage device according to the system described herein.

Figure 9 is a flow chart illustrating storing transmitted data at a remote storage device according to the system described herein.

Figure 10 is a flow chart illustrating steps performed in connection with a local storage device incrementing a sequence number according to a system described herein.

Figure 11 is a schematic diagram illustrating items for constructing and manipulating chunks of data on a local storage device according to an alternative embodiment of the system described herein.

Figure 12 is a flow chart illustrating operation of a host adaptor (HA) in response to a write by a host according to an alternative embodiment of the system described herein.

Figure 13 is a flow chart illustrating transferring data from a local storage device to a remote storage device according to an alternative embodiment of the system described herein.

Figure 14 is a schematic diagram illustrating a plurality of local and remote storage devices with a host according to the system described herein.

Figure 15 is a diagram showing a multi-box mode table used in connection with the system described herein.

Figure 16 is a flow chart illustrating modifying a multi-box mode table according to the system described herein.

Figure 17 is a flow chart illustrating cycle switching by the host according to the system described herein.

Figure 18 is a flow chart illustrating steps performed in connection with a local storage device incrementing a sequence number according to a system described herein.

Figure 19 is a flow chart illustrating transferring data from a local storage device to a remote storage device according to the system described herein.

Figure 20 is a flow chart illustrating transferring data from a local storage device to a remote storage device according to an alternative embodiment of the system described herein.

Figure 21 is a flow chart illustrating providing an active empty indicator message from a remote storage device to a corresponding local storage device according to the system described herein.

Figure 22 is a schematic diagram illustrating a plurality of local and remote storage devices with a plurality of hosts according to the system described herein.

Figure 23 is a flow chart illustrating a processing performed by a remote storage device in connection with data recovery according to the system described herein.

Figure 24 is a flow chart illustrating a processing performed by a hostin connection with data recovery according to the system described herein.

Figure 25 is a schematic diagram showing a storage device, memory, a plurality of directors, and a communication module according to the system described herein.

Figure 26 is a schematic diagram showing a source group, a local destination, and a remote destination according to the system described herein.

Figure 27 is a flow chart illustrating a process performed by a local destination to initialize data recovery parameters according to the system described herein.

Figure 28A is a flow chart illustrating a process performed by a local destination in connection with receiving data during non-failure mode according to the system described herein.

Figure 28B is a flow chart illustrating an alternative process that may be performed by a local destination in connection with receiving data during non-failure mode according to the system described herein.

Figure 28C is a flow chart illustrating an alternative process that may be performed by a local destination in connection with receiving data during non-failure mode according to the system described herein.

Figure 29 is a flow chart illustrating a process performed by a local destination to initialize data recovery parameters according to the system described herein.

Figure 30 is a flow chart illustrating a process performed by a remote destination in connection with collecting failure recovery data according to the system described herein.

Figure 31 is a flow chart illustrating a process performed in connection with failure recovery according to the system described herein.

Figure 32 is a flow chart illustrating processing performed in connection with tenninating ordered writes according to the system describcd herein.

Figure 33 is a flow chart illustrating processing performed in connection with sending data from a local destination to a remote destination according to the system described herein.

Figure 34 is a flow chart illustrating a process performed by a local destination in connection with resetting en-or recovery parameters according to the system described herein.

Figure 35 is a flow chart illustrating a process performed by a remote destination in connection with resetting error recovery parameters according to the system described herein.

Figure 36 is a diagram illustrating a configuration of a source group according to the system described herein.

Figure 37 is a diagram illustrating another configuration of a source group according to the system described herein.

Figure 38 is a flow chart illustrating an alternative embodiment for transferring data from a local storage device to a remote storage device according to the system described herein.

Figure 39 is a schematic diagram illustrating items for constructing and manipulating chunks of data on a remote storage device according to the system described herein.

Figure 40 is a diagram showing a table used to map logical device locations to slots containing data received by a remote storage device according to the system described herein.

Figure 41 is a diagram showing another embodiment of a table used to map logical device locations to slots containing data received by a remote storage device according to the system described herein.

Figure 42 is a schematic diagram showing a source group, a minimal storage local destination, and a remote destination according to an embodiment of the system described herein.

Figure 43 is a schematic diagram showing data storage at a minimal storage local destination according to an embodiment of the system described herein.

Figure 44 is a diagram illustrating a data element used for data storage at a minimal storage local destination according to au embodiment of the system described herein.

Figure 45 is a flow chart illustrating steps performed in connection with a minimal storage local destination receiving data according to an embodiment of the system described herein.

Figure 46 is a flow chart illustrating steps performed in connection with a minimal storage local destination adding data according to an embodiment of the system described herein.

Figure 47 is a flow chart illustrating steps performed in connection with a minimal storage local destination adding data according to another embodiment of the system described herein.

Figure 48 is a flow chart illustrating steps performed in connection with a minimal storage local destination removing data according to an embodiment of the system described herein.

Figure 49 is a flow chart illustrating steps performed in connection with data recovery according to an embodiment of the system described herein.

Figure 50 is a schematic diagram showing a plurality of source groups and remote destinations coupled to a minimal storage local destination according to another embodiment of the system described herein.

Referring to Figure 1, a diagram 20 shows a relationship between a host 22, a local storage device 24 and a remote storage device 26. The host 22 reads and writes data from and to the local storage device 24 via a host adapter (HA) 28, which facilitates the interface between the host 22 and the local storage device 24. Although the diagram 20 only shows one host 22 and one HA 28, it will be appreciated by one of ordinary skill in the art that multiple HA's may be used and that one or more HA's may have one or more hosts coupled thereto.

Data from the local storage device 24 is copied to the remote storage device 26 via an RDF link 29 to cause the data on the remote storage device 26 to be identical to the data on the local storage device 24. Although only the one link 29 is shown, it is possible to have additional links between the storage devices 24, 26 and to have links between one or both of the storage devices 24, 26 and other storage devices (not shown). In addition, the link 29 may be provided using a direct cormection (wired, over-the-air, or some combination thereof), a network (such as the Internet), or any other appropriate means for conveying data. Note that there may be a time delay between the transfer of data from the local storage device 24 to the remote storage device 26, so that the remote storage device 26 may, at certain points in time, contain data that is not identical to the data on the local storage device 24. Communication using RDF is described, for example, in US. Patent No. 5,742,792, which is incorporated by reference herein.

The local storage device 24 includes a first plurality of RDF adapter units (RA's) 30a, 30b, 30c and the remote storage device 26 includes a second plurality of RA's 32a-32c. The RA's 30a-30c, 32a-32c are coupled to the RDF link 29 and are similar to the host adapter 28, but are used to transfer data between the storage devices 24, 26. The software used in connection with the RA's 30a-30c, 32a-32c is discussed in more detail hereinafter.

The storage devices 24, 26 may include one or more disks, each containing a different portion of data stored on each of the storage devices 24, 26. Figure 1 shows the storage device 24 including a plurality of disks 33a, 33b, 33c and the storage device 26 including a plurality of disks 34a, 34b, 34c. The RDF functionality described herein may be applied so that the data for at least a portion of the disks 33a-33c of the local storage device 24 is copied, using RDF, to at least a portion of the disks 34a-34c of the remote storage device 26. Jt is possible that other data of the storage devices 24, 26 is not copied between the storage devices 24, 26, and thus is not identical.

Each of the disks 33a-33c is coupled to a corresponding disk adapter unit (DA) 35a, 35b, 35c that provides data to a corresponding one of the disks 33a-33c and receives data from a corresponding one of the disks 33a-33c. Similarly, a plurality of DA's 36a, 36b, 36c of the remote storage device 26 are used to provide data to corresponding ones of the disks 34a-34c and receive data from corresponding ones of the disks 34a-34c. An internal data path exists between the DA's 35a-35c, the HA 28 and the RA's 30a-30c of the local storage device 24. Similarly, an internal data path exists between the DA's 36a-36c and the RA's 32a-32c of the remote storage device 26.

Note that, in other embodiments, it is possible for more than one disk to be serviced by a DA and that it is possible for more than one DA to service a disk.

The local storage device 24 also includes a global memory 37 that may be used to facilitate data transferred between the DA's 35a-35c, the HA 28 and the RA's 30a- 30c. The memory 37 may contain tasks that are to be performed by one or more of the DA's 35a-35c, the I-lA 28 and the RA's 30a-30c, and a cache for data fetched from one or more of the disks 33a-33c. Similarly, the remote storage device 26 includes a global memory 38 that may contain tasks that are to be performed by one or more of the DA's 36a-36c and the RA's 32a-32c, and a cache for data fetched from one or more of the disks 34a-34c. Use of the memories 37, 38 is described in more detail hereinafter.

The storage space in the local storage device 24 that corresponds to the disks 33a-33c may be subdivided into a plurality of volumes or logical devices. The logical devices may or may not correspond to the physical storage space of the disks 33a-33c.

Thus, for example, the disk 33a may contain a plurality of logical devices or, alternatively, a single logical device could span both of the disks 33a, 33b. Similarly, the storage space for the remote storage device 26 that comprises the disks 34a-34c may be subdivided into a plurality of volumes or logical devices, where each of the logical devices may or may not corresijond to one or more of the disks 34a-34c.

Providing an RDF mapping between portions of the local storage device 24 and the remote storage device 26 involves setting up a logical device on the remote storage device 26 that is a remote mirror for a logical device on the local storage device 24.

The host 22 reads and writes data from and to the logical device on the local storage device 24 and the RDF mapping causes modified data to be transferred from the local storage device 24 to the remote storage device 26 using the RA's, 30a-30c, 32a-32c and the RDF link 29. In steady state operation, the logical device on the remote storage device 26 contains data that is identical to the data of the logical device on the local storage device 24. The logical device on the local storage device 24 that is accessed by the host 22 is referred to as the "RI volume" (or just "RI") while the logical device on the remote storage device 26 that contains a copy of the data on the Ri volume is called the "R2 volume" (or just "R2"). Thus, the host reads and writes data from and to the Ri volume and RDF handles automatic copying and updating of the data from the Ri volume to the R2 volume. The system described herein may be implemented using software, hardware, and/or a combination of software and hardware where software may be stored in an appropriate storage medium and executed by one or more processors.

Referring to Figure 2, a path of data is illustrated from the host 22 to the local storage device 24 and the remote storage device 26. Data written from the host 22 to the local storage device 24 is stored locally, as illustrated by the data element 51 of the local storage device 24. The data that is written by the host 22 to the local storage device 24 is also maintained by the local storage device 24 in connection with being sent by the local storage device 24 to the remote storage device 26 via the link 29.

In the system described herein, each data write by the host 22 (of, for example a record, a plurality of records, a track, etc.) is assigned a sequence number. The sequence number may be provided in an appropriate data field associated with the write. In Figure 2, the writes by the host 22 are shown as being assigned sequence number N. All of the writes performed by the host 22 that are assigned sequence number N are collected in a single chunk of data 52. The chunk 52 represents a plurality of separate writes by the host 22 that occur at approximately the same time.

Generally, the local storage device 24 accumulates chunks of one sequence number while transmitting a previously accumulated chunk (having the previous sequence number) to the remote storage device 26. Thus, while the local storage device 24 is accumulating writes from the host 22 that are assigned sequence number N, the writes that occurred for the previous sequence number (N-I) are transmitted by the local storage device 2410 the remote storage device 26 via the link 29. A chunk 54 represents writes from the host 22 that were assigned the sequence number N-i that have not been transmitted yet to the remote storage device 26.

The remote storage device 26 receives the data from the chunk 54 corresponding to writes assigned a sequence number N-I and constructs a new chunk 56 of host writes having sequence number N-I. The data may be transmitted using appropriate RDF protocol that acknowledges data sent across the link 29. When the remote storage device 26 has received all of the data from the chunk 54, the local storage device 24 sends a commit message to the remote storage device 26 to commit all the data assigned the N-I sequence number corresponding to thc chunk 56.

Generally, once a chunk corresponding to a particular sequence number is committed, that chunk may be written to the logical storage device. This is illustrated in Figure 2 with a chunk 58 corresponding to writes assigned sequence number N-2 (i.e., two before the current sequence number being used in connection with writes by the host 22 to the local storage device 26). In Figure 2, the chunk 58 is shown as being written to a data element 62 representing disk storage for the remote storage device 26. Thus, the remote storage device 26 is receiving and accumulating the chunk 56 corresponding to sequence number N-I while the chunk 58 corresponding to the previous sequence number (M-2) is being written to disk storage of the remote storage device 26 illustrated by the data element 62. In some embodiments, the data for the chunk 58 is marked for write (but not necessarily written immediately), while the data for the chunk 56 is not.

Thus, in operation, the host 22 writes data to the local storage device 24 that is stored locally in the data element 51 and is accumulated in the chunk 52. Once all of the data for a particular sequence number has been accumulated (described elsewhere herein), the local storage device 24 increments the sequence number. Data from the chunk 54 corresponding to one less than the current sequence number is transferred from the local storage device 24 to the remote storage device 26 via the link 29. The chunk 58 corresponds to data for a sequence number that was committed by the local storage device 24 sending a message to the remote storage device 26. Data from the chunk 58 is written to disk storage of the remote storage device 26.

Note that the writes within a particular one of the chunks 52, 54, 56, 58 are not IS necessarily ordered. However, as described in more detail elsewhere herein, every write for the chunk 58 corresponding to sequence number N-2 was begun prior to beginning any of the writes for the chunks 54, 56 corresponding to sequence number N- 1. In addition, every write for the chunks 54, 56 corresponding to sequence number N- 1 was begun prior to beginning any of the writes for the chunk 52 corresponding to sequence number N. Thus, in the event of a communication failure between the local storage device 24 and the remote storage device 26, the remote storage device 26 may simply finish writing the last committed chunk of data (the chunk 58 in the example of Figure 2) and can be assured that the state of the data at the remote storage device 26 is ordered in the sense that the data element 62 contains all of the writes that were begun prior to a certain point in time and contains no writes that were begun after that point in time. Thus, R2 always contains a point in time copy of Ri and it is possible to reestablish a consistent image from the R2 device.

Referring to Figure 3, a diagram 70 illustrates items used to construct and maintain the chunks 52, 54. A standard logical device 72 contains data written by the host 22 and corresponds to the data element 51 of Figure 2 and the disks 33a-33c of Figure 1. The standard logical device 72 contains data written by the host 22 to the local storage device 24.

Two linked lists of pointers 74, 76 are used in connection with the standard S logical device 72. The linked lists 74, 76 correspond to data that may be stored, for example, in the memory 37 of the local storage device 24. The linked list 74 contains a plurality of pointers 81-85, each of which points to a slot of a cache 88 used in connection with the local storage device 24. Similarly, the linked list 76 contains a plurality of pointers 91-95, eachof which points to a slot of the cache 88. In some embodiments, the cache 88 may be provided in the memory 37 of the local storage device 24. The cache 88 contains a plurality of cache slots 102-104 that may be used in connection to writes to the standard logical device 72 and, at the same time, used in connection with the linked lists 74, 76.

Each of the linked lists 74, 76 may be used for one of the chunks of data 52, 54 so that, for example, the linked list 74 may correspond to the chunk of data 52 for sequence number N while the linked list 76 may correspond to the chunk of data 54 for sequence number N-I. Thus, when data is written by the host 22 to the local storage device 24, the data is provided to the cache 88 and, in some cases (described elsewhere herein), an appropriate pointer of the linked list 74 is created. Note that the data will not be removed from the cache 88 until the data is destaged to the standard logical device 72 and the data is also no longer pointed to by one of the pointers 81-85 of the linked list 74, as described elsewhere herein.

In an embodiment herein, one of the linked lists 74, 76 is deemed "active" while the other is deemed "inactive". Thus, for example, when the sequence number N is even, the linked list 74 may be active while the linked list 76 is inactive. The active one of the linked lists 74, 76 handles writes from the host 22 while the inactive one of the linked lists 74, 76 corresponds to the data that is being transmitted from the local storage device 24 to the remote storage device 26.

While the data that is written by the host 22 is accumulated using the active one of the linked lists 74, 76 (for the sequence number N), the data corresponding to the inactive one of the linked lists 74, 76 (for previous sequence number N-i) is transmitted from the local storage device 24 to the remote storage device 26. The RA's 30a-30c use the linked lists 74, 76 to determine the data to transmit from the local storage device 24 to the remote storage device 26.

Once data corresponding to a particular one of the pointers in one of the linked lists 74, 76 has been transmitted to the remote storage device 26, the particular one of the pointers may be removed from the appropriate one of the linked lists 74, 76. In addition, the data may also be marked for removal from the cache 88 (i.e., the slot may be returned to a pool of slots for later, unrelated, use) provided that the data in the slot is not otherwise needed for another purpose (e.g., to be destaged to the standard logical device 72). A mechanism may be used to ensure that data is not removed from the cache 88 until all devices are no longer using the data. Such a mechanism is described, for example, in U.S. Patent No. 5,537,568 issued on July 16, 1996 and in U.S. Patent No. 6,594,742 issued on July 15, 2003, both of which are incorporated by reference herein.

Referring to Figure 4, a slot 120, like one of the slots 102-104 of the cache 88, includes a header 122 and data 124. The header 122 corresponds to overhead information used by the system to manage the slot 120. The data 124 is the corresponding data from the disk that is being (temporarily) stored in the slot 120.

Information in the header 122 includes pointers hack to the disk, tilTie stamp(s), etc. The header 122 also includes a cache stamp 126 used in connection with the system described herein. In an embodiment herein, the cache stamp 126 is eight bytes.

Two of the bytes are a "password" that indicates whether the slot 120 is being used by the system described herein. In other embodiments, the password may be one byte while the following byte is used for a pad. As described elsewhere herein, the two bytes of the password (or one byte, as the case may be) being equal to a particular value indicates that the slot 120 is pointed to by at least one entry of the linked lists 74, 76.

The password not being equal to the particular value indicates that the slot 120 is not pointed to by an entry of the linked lists 74, 76. Use of the password is described elsewhere herein.

The cache stamp 126 also includes a two byte field indicating the sequence number (e.g., N, N-i, N-2, etc.) of the data 124 of the slot 120. As described elsewhere herein, the sequence number field of the cache stamp 126 may be used to facilitate the processing described herein. The remaining four bytes of the cache stamp 126 may be used for a pointer, as described elsewhere herein. Of course, the two bytes of the sequence number and the four bytes of the pointer are only valid when the password equals the particular value that indicates that the slot 120 is pointed to by at least one entry in one of the lists 74, 76.

Referring to Figure 5, a flow chart 140 illustrates steps performed by the HA 28 in connection with a host 22 performing a write operation. Of course, when the host 22 performs a write, processing occurs for handling the write in a normal fashion irrespective of whether the data is part of an RI /R2 RDF group. For example, when the host 22 writes data for a portion of the disk, the write occurs to a cache slot which is eventually destaged to the disk. The cache slot may either be a new cache slot or may be an already existing cache slot created in connection with a previous read and/or write operation to the same track.

Processing begins at a first step 142 where a slot corresponding to the write is locked. In an embodiment herein, each of the slots 102- 104 of the cache 88 corresponds to a track of data on the standard logical device 72. Locking the slot at the step 142 prevents additional processes from operating on the relevant slot during the processing performed by the HA 28 corresponding to the steps of the flow chart 140.

Following step 142 is a step 144 where a value for N, the sequence number, is set. As discussed elsewhere herein, the value for the sequence number obtained at the step 144 is maintained during the entire write operation performed by the HA 28 while the slot is locked. As discussed elsewhere herein, the sequence number is assigned to each write to set the one of the chunks of data 52, 54 to which the write belongs.

Writes performed by the host 22 are assigned the current sequence number. It is useful that a single write operation maintain the same sequence number throughout.

Following the step 144 is a test step 146 which determines if the password field of the cache slot is valid. As discussed above, the system described herein sets the password ficid to a predetermined value to indicate that the cache slot is already in one of the linked lists of pointers 74, 76. If it is determined at the test step 146 that the password field is not valid (indicating that the slot is new and that no pointers from the lists 74, 76 point to the slot), then control passes from the step 146 to a step 148, where the cache stamp of the new slot is set by setting the password to the predetermined value, setting the sequence number field to N, arid setting the pointer field to Null. In other embodiments, the pointer field may be set to point to the slot itself.

Following the step 148 is a step 152 where a pointer to the new slot is added to the active one of the pointer lists 74, 76. In an embodiment herein, the lists 74, 76 are circular doubly linked lists, and the new pointer is added to the circular doubly linked list in a conventional fashion. Of course, other appropriate data structures could be used to manage the lists 74, 76. Following the step 152 is a step 154 where flags are set. At the step 154, the RDF_WP flag (RDF write pending flag) is set to indicate that the slot needs to be transmitted to the remote storage device 26 using RDF. In addition, at the step 154, the IN CACHE flag is set to indicate that the slot needs to be destaged to the standard logical device 72. Following the step 154 is a step 156 where the data being written by the host 22 and the HA 28 is written to the slot. Following the step 156 is a step 158 where the slot is unlocked. Following step 158, processing is complete.

If it is determined at the test step 146 that the password field of the slot is valid (indicating that the slot is aleady pointed to by at least one pointer of the lists 74, 76), then control transfers from the step 146 to a lest step 162, where it is determined whether the sequence number field of the slot is equal to the current sequence number, N. Note that there are two valid possibilities for the sequence number field of a slot with a valid password. It is possible for the sequence number field to be equal to N, the current sequence number. This occurs when the slot corresponds to a previous write with sequence number N. The other possibility is for the sequence number field to equal N-i. This occurs when the slot corresponds to a previous write with sequence S number N-i. Any other value for the sequence number field is invalid. Thus, for some embodiments, it may be possible to include error/validity checking in the step 162 or possibly make error/validity checking a separate step. Such an error may be handled in any appropriate fashion, which may include providing a message to a user.

If it is determined at the step 162 that the value in the sequence number field of the slot equals the current sequence number N, then no special processing is required and control transfers from the step 162 to the step 156, discussed above, where the data is written to the slot. Otherwise, if the value of the sequence number field is N-I (the only other valid value), then control transfers from the step 162 to a step 164 where a new slot is obtained. The new slot obtained at the step 164 may be used to store the data being written.

Following the step 164 is a step 166 where the data from the old slot is copied to the new slot that was obtained at the step 164. Note that that the copied data includes the RDF_WP flag, which should have been set at the step 154 on a previous write when the slot was first created. Following the step 166 is a step 168 where the cache stamp for the new slot is set by setting the password field to the appropriate value, setting the sequence number field to the current sequence number, N, and setting the pointer field to point to the old slot. Following the step 168 is a step 172 where a pointer to the new slot is added to the active one of the linked lists 74, 76. Following the step 172 is the step 156, discussed above, where the data is written to the slot which, in this case, is the new slot.

Referring to Figure 6, a flow chart 200 illustrates steps performed in connection with the RAs 30a-30c scanning the inactive one of the lists 72,74 to transmit RDF data from the local storage device 24 to the remote storage device 26. As discussed above, the inactive one of the lists 72, 74 points to slots corresponding to the N-I cycle for the RI device when the N cycle is being written to the RI device by the host using the active one of the lists 72, 74.

Processing begins at a first step 202 where it is determined if there are any entries in the inactive one of the lists 72, 74. As data is transmitted, the corresponding entries are removed from the inactive one of the lists 72, 74. In addition, new writes are provided to the active one of the lists 72, 74 and not generally to the inactive one of the lists 72, 74. Thus, it is possible (and desirable, as described elsewhere herein) for (he inactive one of the lists 72, 74 to contain no data at certain times. If it is determined at the step 202 that there is no data to be transmitted, then the inactive one of the lists 72, 74 is continuously polled until data becomes available. Data for sending becomes available in connection with a cycle switch (discussed elsewhere herein) where the inactive one of the lists 72, 74 becomes the active one of the lists 72, 74, and vice versa.

If it is determined at the step 202 that there is data available for sending, control transfers from the step 202 to a step 204, where the slot is verified as being correct.

The processing performed at the step 204 is an optional "sanity check" that may include verifying that the password field is correct and verifying that the sequence number field is correct. If there is incorrect (unexpected) data in the slot, error processing may be performed, which may include notifying a user of the error and possibly error recovery processing.

Following the step 204 is a step 212, where the data is sent via RDF in a conventional fashion. In an embodiment herein, the entire slot is not transmitted.

Rather, only records within the slot that have the appropriate mirror bits set (indicating the records have changed) are transmitted to the remote storage device 26. However, in other embodiments, it may be possible to transmit the entire slot, provided that the remote storage device 26 only writes data corresponding to records having appropriate mirror bits set and ignores other data for the track, which may or may not be valid.

Following the step 212 is a test step 214 where it is determined if the data that was transmitted has been acknowledged by the R2 device. If not, the data is resent, as indicated by the flow from the step 214 back to the step 212. In other embodiments, different and more involved processing may used to send data and acknowledge receipt thereof. Such processing may include error reporting and alternative processing that is performed after a certain number of attempts to send the data have failed Once it is determined at the test step 214 that the data has been successfully sent, control passes from the step 214 to a step 216 to clear the RDF_WP flag (since the data has been successfiully sent via RDF). Following the step 216 is a test step 218 where it is determined if the slot is a duplicate slot created in connection with a write to a slot already having an existing entry in the inactive one of the lists 72, 74. This possibility is discussed above in connection with the steps 162, 164, 166, 168, 172. If it is determined at the step 218 that the slot is a duplicate slot, then control passes from the step 218 to a step 222 where the slot is returned to the pooi of available slots (to be reused). In addition, the slot may also be aged (or have some other appropriate mechanism applied thereto) to provide for immediate reuse ahead of other slots since the data provided in the slot is not valid for any other purpose. Following the step 222 or the step 218 if the slot is not a duplicate slot is a step 224 where the password field of the slot header is cleared so that when the slot is reused, the test at the step 146 of Figure 5 properly classifies the slot as a new slot.

Following the step 224 is a step 226 where the entry in the inactive one of the lists 72, 74 is removed. Following the step 226, control transfers back to the step 202, discussed above, where it is determined if there are additional entries on the inactive one of the lists 72, 74 corresponding to data needing to be transferred.

Referring to Figure 7, a diagram 240 illustrates creation and manipulation of the chunks 56, 58 used by the remote storage device 26. Data that is received by the remote storage device 26, via the link 29, is provided to a cache 242 of the remote storage device 26. The cache 242 may be provided, for example, in the memory 38 of the remote storage device 26. The cache 242 includes a plurality of cache slots 244- 246, each of which may be mapped to a track of a standard logical storage device 252.

The cache 242 is similar to the cache 88 of Figure 3 and may contain data that can be destaged to the standard logical storage device 252 of the remote storage device 26.

The standard logical storage device 252 corresponds to the data element 62 shown in Figure 2 and the disks 34a-34c shown in Figure 1.

The remote storage device 26 also contains a pair of cache only virtual devices 254, 256. The cache only virtual devices 254, 256 corresponded device tables that may be stored, for example, in the memory 38 of the remote storage device 26. Each track entry of the tables of each of the cache only virtual devices 254, 256 point to either a track of the standard logical device 252 or point to a slot of the cache 242. Cache only virtual devices are described in a copending U.S. patent application titled CACHE-ONLY VIRTUAL DEVICES, filed on March 25, 2003 and having serial no. -l O/396,00, which is incorporated by reference herein.

The plurality of cache slots 244-246 may be used in connection to writes to the standard logical device 252 and, at the same time, used in connection with the cache only virtual devices 254, 256. In an embodiment herein, each of track table entries of the cache only virtual devices 254, 256 contain a null to indicate that the data for that track is stored on a corresponding track of the standard logical device 252. Otherwise, an entry in the track table for each of the cache only virtual devices 254, 256 contains a pointer to one of the slots 244-246 in the cache 242.

Each of the cache only virtual devices 254, 256 corresponds to one of the data chunks 56, 58. Thus, for example, th&cache only virtual device 254 may correspond to the data chunk 56 while the cache only virtual device 256 may correspond to the data chunk 58. In an embodiment herein, one of the cache only virtual devices 254, 256 may be deemed "active" while the other one of the cache only virtual devices 254, 256 may be deemed "inactive". The inactive one of the cache only virtual devices 254, 256 may correspond to data being received from the local storage device 24 (i.e., the chunk 56) while the active one of the cache only virtual device 254, 256 corresponds to data being restored (written) to the standard logical device 252.

Data from the local storage device 24 that is received via the link 29 may be placed in one of the slots 244-246 of the cache 242. A corresponding pointer of the inactive one of the cache only virtual devices 254, 256 may be set to point to the received data. Subsequent data having the same sequence number may be processed in a similar manner. At some point, the local storage device 24 provides a message conunitting all of the data sent using the same sequence number. Once the data for a particular sequence number has been committed, the inactive one of the cache only virtual devices 254, 256 becomes active and vice versa. At that point, data from the now active one of the cache only virtual devices 254, 256 is copied to the standard logical device 252 while the inactive one of the cache only virtual devices 254, 256 is used to receive new data (having a new sequence number) transmitted from the local storage device 24 to the remote storage device 26.

As data is removed from the active one of the cache only virtual devices 254, 256 (discussed elsewhere herein), the corresponding entry in the active one of the cache only virtual devices 254, 256 may be set to null. In addition, the data may also be removed from the cache 244 (i.e., the slot returned to the pool of free slots for later use) provided that the data in the slot is not otherwise needed for another purpose (e.g., to be destaged to the standard logical device 252). A mechanism may be used to ensure that data is not removed from the cache 242 until all mirrors (including the cache only virtual devices 254, 256) are no longer using the data. Such a mechanism is described, for example, in U.S. Patent No. 5,537,568 issued on July 16, 1996 and in U.S. Patent No. 6,594,742 issued on July 15, 2003, both of which are incorporated by reference herein.

In some embodiments discussed elsewhere herein, the remote storage device 26 may maintain linked lists 258, 262 like the lists 74, 76 used by the local storage device 24. The lists 258, 262 may contain information that identifies the slots of the corresponding cache only virtual devices 254, 256 that have been modified, where one of the lists 258, 262 corresponds to one of the cache only virtual devices 254, 256 and the other one of the lists 258, 262 corresponds to the other one of the cache only virtual devices 254, 256. As discussed elsewhere herein, the lists 258, 262 may be used to facilitate restoring data from the cache only virtual devices 254, 256 to the standard logical device 252.

Referring to Figure 8, a flow chart 270 illustrates steps performed by the remote storage device 26 in connection with processing data for a sequence number commit transmitted by the local storage device 24 to the remote storage device 26. As discussed elsewhere herein, the local storage device 24 periodically increments sequence numbers. When this occurs, the local storage device 24 finishes transmitting all of the data for the previous sequence number and then sends a commit message for the previous sequence number.

Processing begins at a first step 272 where the conmiit is received. Following the step 272 is a test step 274 which determines if the active one of the cache only virtual devices 254, 256 of the remote storage device 26 is empty. As discussed elsewhere herein, the inactive one of the cache only virtual devices 254, 256 of the remote storage device 26 is used to accumulate data from the local storage device 24 sent using RDF while the active one of the cache only virtual devices 254, 256 is restored to the standard logical device 252.

If it is determined at the test step 274 that the active one of the cache only virtual devices 254, 256 is not empty, then control transfers from the test step 274 to a step 276 where the restore for the active one of the cache only virtual devices 254, 256 is completed prior to further processing being performed. Restoring data from the active one of the cache only virtual devices 254, 256 is described in more detail elsewhere herein. It is useful that the active one of the cache only virtual devices 254, 256 is empty prior to handling the commit and beginning to restore data for the next sequence number.

Following the step 276 or following the step 274 if the active one of the cache only virtual devices 254, 256 is determined to be empty, is a step 278 where the active one of the cache only virtual devices 254, 256 is made inactive. Following the step 278 is a step 282 where the previously inactive one of the cache only virtual devices 254, 256 (i.e., the one that was inactive prior to execution of the step 278) is made active.

Swapping the active and inactive cache only virtual devices 254, 256 at the steps 278, 282 prepares the now inactive (and empty) one of the cache only virtual devices 254, 256 to begin to receive data from the local storage device 24 for the next sequence number. -Following the step 282 is a step 284 where the active one of the cache only virtual devices 254, 256 is restored to the standard logical device 252 of the remote storage device 26. Restoring the active one of the cache only virtual devices 254, 256 to the standard logical device 252 is described in more detail hereinafter. However, note that, in some embodiments, the restore process is begun, but not necessarily completed, at the step 284. Following the step 284 is a step 286 where the commit that was sent from the local storage device 24 to the remote storage device 26 is acknowledged back to the local storage device 24 so that the local storage device 24 is informed that the commit was successful. Following the step 286, processing is complete.

Referring to Figure 9, a flow chart 300 illustrates in more detail the steps 276, 284 of Figure 8 where the remote storage device 26 restores the active one of the cache only virtual devices 254, 256. Processing begins at a first step 302 where a pointer is set to point to the first slot of the active one of the cache oniy virtual devices 254, 256.

The pointer is used to iterate through each track table entry of the active one of the cache only virtual devices 254, 256, each of which is processed individually.

Following the step 302 is a test step 304 where it is determined if the track of the active one of the cache only virtual devices 254, 256 that is being processed points to the standard logical device 252. If so, then there is nothing to restore. Otherwise, control transfers from the step 304 to a step a 306 where the corresponding slot of the active one of the cache only virtual devices 254, 256 is locked.

Following the step 306 is a test step 308 which determines if the corresponding slot of the standard logical device 252 is already in the cache of the remote storage device 26. If so then control transfers from the test step 308 to a step 312 where the slot of the standard logical device is locked. Following step 312 is a step 314 where the data from the active one of the cache only virtual devices 254, 256 is merged with the data in the cache for the standard logical device 252. Merging the data at the step 314 involves overwriting the data for the standard logical device with the new data of the active one of the cache only virtual devices 254, 256. Note that, in embodiments thai provide for record level flags, it may be possible to simply OR the new records from the active one of the cache only virtual devices 254, 256 to the records of the standard logical device 252 in the cache. That is, if the records are interleaved, then it is only necessary to use the records from the active one of the cache only virtual devices 254, 256 that have changed and provide the records to the cache slot of the standard logical device 252. Following step 314 is a step 316 where the slot of the standard logical device 252 is unlocked. Following step 316 is a step 318 where the slot of the active one of the cache only virtual devices 254, 256 that is being processed is also unlocked.

If it is determined at the test step 308 that the corresponding slot of the standard logical device 252 is not in cache, then control transfers from the test step 308 to a step 322 where the track entry for the slot of the standard logical device 252 is changed to indicate that the slot of the standard logical device 252 is in cache (e.g., an IN_CACHE flag may be set) and needs to be destaged. As discussed elsewhere herein, in some embodiments, only records of the track having appropriate mirror bits set may need to be destaged. Following the step 322 is a step 324 where a flag for the track may be set to indicate that the data for the track is in the cache.

Following the step 324 is a step 326 where the slot pointer for the standard logical device 252 is changed to point to the slot in the cache. Following the step 326 is a test step 328 which determines if the operations performed at the steps 322, 324, 326 have been successful. In some instances, a single operation called a "compare and swap" operation may be used to perform the steps 322, 324, 326. If these operations are not successful for any reason, then control transfers from the step 328 back to the step 308 to reexamine if the corresponding track of the standard logical device 252 is in the cache. Otherwise, if it is determined at the test step 328 that the previous operations have been successful, then control transfers from the test step 328 to the step 318, discussed above.

Following the step 318 is a test step 332 which determines if the cache slot of the active one of the cache only virtual devices 254, 256 (which is being restored) is still being used. in some cases, it is possible that the slot for the active one of the cache only virtual devices 254, 256 is still being used by another mirror. If it is determined at the test step 332 that the slot of the cache only virtual device is not being used by another mirror, then control transfers from the test step 332 to a step 334 where the slot is released for use by other processes (e.g., restored to pooi of available slots, as discussed elsewhere herein). Following the step 334 is a step 336 to point to the next slot to process the next slot of the active one of the cache only virtual devices 254, 256.

Note that the step 336 is also reached from the test step 332 if it is determined at the step 332 that the active one of the cache only virtual devices 254, 256 is still being used by another mirror. Note also that the step 336 is reached from the test step 304 if it is determined at the step 304 that, for the slot being processed, the active one of the cache only virtual devices 254, 256 points to the standard logical device 252. Following the step 336 is a test step 338 which determines if there are more slots of the active one of the cache only virtual devices 254, 256 to be processed. If not, processing is complete.

Otherwise, control transfers from the test step 338 back to the step 304.

In another embodiment, it is possible to construct lists of modified slots for the received chunk of data 56 corresponding to the N-i cycle on the remote storage device 26, such as the lists 258, 262 shown in Figure 7. As the data is received the remote storage device 26 constructs a linked list of modified slots. The lists that are constructed may be circular, linear (with a NULL termination), or any other appropriate design. The lists may then be used to restore the active one of the cache only virtual devices 254, 256.

lime flow chart 300 of Figure 9 shows two alternative paths 342, 344 that illustrate operation of embodiments where a list of modified slots is used. At the step 302, a pointer (used for iterating through the list of modified slots) is made to point to -34 the first element of the list. Following the step 302 is the step 306, which is reached by the alternative path 342. In embodiments that use lists of modi lied slots, the test step 304 is not needed since no slots on the list should point to the standard logical device 252.

Following the step 306, processing continues as discussed above with the previous embodiment, except that the step 336 refers to traversing the list of modified slots rather than pointing to the next slot in the COVD. Similarly, the test at the step 338 determines if the pointer is at the end of the list (or back to the begiiming in the case of a circular linked list). Also, if it is determined at the step 338 that there are more slots to process, then control transfers from the step 338 to the step 306, as illustrated by the alternative path 344. As discussed above, for embodiments that use a list of modified slots, the step 304 may be eliminated.

Referring to Figure 10, a flow chart 350 illustrates steps performed in connection with the local storage device 24 increasing the sequence number.

Processing begins at a first step 352 where the local storage device 24 waits at least M seconds prior to increasing the sequence number. In an embodiment herein, M is thirty, but of course M could be any number. Larger values for M increase the amount of data that may be lost if communication between the storage devices 24, 26 is disrupted.

However, smaller values for M increase the total amount of overhead caused by incrementing the sequence number more frequently.

Following the step 352 is a test step 354 which determines if all of the 1-lA's of the local storage device 24 have set a bit indicating that the HA's have completed all of the 110's for a previous sequence number. When the sequence number changes, each of the HA's notices the change and sets a bit indicating that all I/O's of the previous sequence number are completed. For example, if the sequence number changes from N- i to N, an HA will set the bit when the HA has completed all I/O's for sequence number N-i. Note that, in some instances, a single 110 for an HA may take a long time and may still be in progress even after the sequence number has changed. Note also that, for some systems, a different mechanism may be used to determine if all of the I-IA's have completed their N-I 110's. The different mechanism may include examining device tables in the memory 37.

If it is determined at the test step 354 that 110's from the previous sequence number have been completed, then control transfers from the step 354 to a test step 356 which determines if the inactive one of the lists 74, 76 is empty. Note that a sequence number switch may not be made unless and until all of the data corresponding to the inactive one of the lists 74, 76 has been completely transmitted from the local storage device 24 to the remote storage device 26 using the RDF protocol. Once the inactive one of the lists 74, 76 is determined to be empty, then control transfers from the step 356 to a step 358 where the commit for the previous sequence number is sent from the local storage device 24 to the remote storage device 26. As discussed above, the remote storage device 26 receiving a commit message for a particular sequence number will cause the remote storage device 26 to begin restoring the data corresponding to the sequence number.

Following the step 358 is a step 362 where the copying of data for the inactive one of the lists 74, 76 is suspended. As discussed elsewhere herein, the inactive one of the lists is scanned to send corresponding data from the local storage device 24 to the remote storage device 26. It is useful to suspend copying data until the sequence number switch is completed. In an embodiment herein, the suspension is provided by sending a message to the RA's 30a-30c. However, it will be appreciated by one of ordinary skill in the art that for embodiments that use other components to facilitate sending data using the system described herein, suspending copying may be provided by sending appropriate messages/commands to the other components.

Following step 362 is a step 364 where the sequence number is incremented.

Following step 364 is a step 366 where the bits for the HA's that are used in the test step 354 are all cleared so that the bits may be set again in connection with the increment of the sequence number. Following step 366 is a test step 372 which determines if the remote storage device 26 has ackowledged the commit message sent at the step 358. Acknowledging the commit message is discussed above in connection with Figure 8. Once it is determined that the remote storage device 26 has acknowledged the commit message sent at the step 358, control transfers from the step 372 to a step 374 where the suspension of copying, which was provided at the step 362, is cleared so that copying may resume. Following step 374, processing is complete.

Note that it is possible to go from the step 374 back to the step 352 to begin a new cycle to continuously increment the sequence number.

It is also possible to use COVD's on the Ri device to collect slots associated with active data and inactive chunks of data. In that case, just as with the R2 device, one CO\TD could be associated with the inactive sequence nunTher and another COVD could be associated with the active sequence number. This is described below.

Referring to Figure 11, a diagram 400 illustrates items used to construct and maintain the chunks 52, 54. A standard logical device 402 contains data written by the host 22 and corresponds to the data element 51 of Figure 2 and the disks 33a-33c of Figure 1. The standard logical device 402 contains data written by the host 22 to the local storage device 24.

Two cache only virtual devices 404, 406 arc used in connection with the standard logical device 402. The cache only virtual devices 404, 406 corresponded device tables that may be stored, for example, in the memory 37 of the local storage device 24. Each track entry of the tables of each of the cache only virtual devices 404, 406 point to either a track of the standard logical device 402 or point to a slot of a cache 408 used in connection with the local storage device 24. In some embodiments, the cache 408 may be provided in the memory 37 of the local storage device 24.

The cache 408 contains a plurality of cache slots 412-414 that may be used in connection to writes to the standard logical device 402 and, at the same time, used in connection with the cache only virtual devices 404, 406. Jn an embodiment herein, each track table entry of the cache only virtual devices 404, 406 contains a null to point to a corresponding track of the standard logical device 402. Otherwise, an entry in the track table for each of the cache only virtual devices 404, 406 contains a pointer to one of the slots 412-414 in the cache 408.

Each of the cache only virtual devices 404, 406 may be used for one of the chunks of data 52, 54 so that, for example, the cache only virtual device 404 may correspond to the chunk of data 52 for sequence number N while the cache only virtual device 406 may correspond to the chunk of data 54 for sequence number N-I. Thus, when data is written by the host 22 to the local storage device 24, the data is provided to the cache 408 and an appropriate pointer of the cache only virtual device 404 is adjusted. Note that the data will not be removed from the cache 408 until the data is destaged to the standard logical device 402 and the data is also released by the cache only virtual device 404, as described elsewhere herein.

In an embodiment herein, one of the cache only virtual devices 404, 406 is deemed "active" while the other is deemed "inactive". Thus, for example, when the sequence number N is even, the cache only virtual device 404 may be active while the cache only virtual device 406 is inactive. The active one of the cache only virtual devices 404, 406 handles writes from the host 22 while the inactive one of the cache only virtual devices 404, 406 corresponds to the data that is being transmitted from the local storage device 24 to the remote storage device 26.

While the data that is written by the host 22 is accumulated using the active one of the cache only virtual devices 404, 406 (for the sequence number N), the data corresponding to the inactive one of the cache only virtual devices 404, 406 (for previous sequence number N-i) is transmitted from the local storage device 24 to the remote storage device 26. For this and related embodiments, the DA's 35a-35c of the local storage device handle scanning the inactive one of the cache only virtual devices 404, 406 to send copy requests to one or more of the RA's 30a-30c to transmit the data from the local stOrage device 24 to the remote storage device 26. Thus, the steps 362, 374, discussed above in connection with suspending and resuming copying, may include providing messages/commands to the DA's 35a-35c.

Once the data has been transmitted to the remote storage device 26, thecorresponding entry in the inactive one of the cache oniy virtual devices 404, 406 may be set to null. In addition, the data may also be removed from the cache 408 (i.e., the slot returned to the pooi of slots for later use) if the data in the slot is not otherwise needed for another purpose (e.g., to be destaged to the standard logical device 402). A mechanism may be used to ensure that data is not removed from the cache 408 until all mirrors (including the cache only virtual devices 404, 406) are no longer using the data.

Such a mechanism is described, for example, in U.S. Patent No. 5,537,568 issued on July 16, 1996 and in U.S. Patent No. 6,594,742 issued on July 15, 2003, both of which are incorporated by reference herein.

Referring to Figure 12, a flow chart 440 illustrates steps performed by the HA 28 in connection with a host 22 performing a write operation for embodiments where two COVD's are used by the RI device to provide the system described herein.

Processing begins at a first step 442 where a slot corresponding to the write is locked.

In an embodiment herein, each of the slots 4 12-414 of the cache 408 corresponds to a track of data on the standard logicaidevice 402. Locking the slot at the step 442 prevents additional processes from operating on the relevant slot during the processing performed by the HA 28 corresponding to the steps of the flow chart 440.

Following the step 442 is a step 444 where a value for N, the sequence number, is set. Just as with the embodiment that uses lists rather than COVD's on the Ri side, the value for the sequence number obtained at the step 444 is maintained during the entire write operation performed by the HA 28 while the slot is locked. As discussed elsewhere herein, the sequence number is assigned to each write to set the one of the chunks of data 52, 54 to which the write belongs. Writes performed by the host 22 are assigned the current sequence number. It is useful that a single write operation maintain the same sequence number throughout.

Following the step 444 is a test step 446, which determines if the inactive one of the cache only virtual devices 404, 406 already points to the slot that was locked at the step 442 (the slot being operated upon). This may occur if a write to the same slot was provided when the sequence number was one less than the current sequence number.

The data corresponding to the write for the previous sequence number may not yet have been transmitted to the remote storage device 26.

if it is determined at the test step 446 that the inactive one of the cache only virtual devices 404, 406 does not point to the slot, then control transfers from the test step 446 to another lest step 448, where it is determined if the active one of the cache only virtual devices 404, 406 points to the slot, it is possible for the active one of the cache only virtual devices 404, 406 to point to the slot if there had been a previous write to the slot while the sequence number was the same as the current sequence number. If it is determined at the test step 448 that the active one of the cache only virtual devices 404, 406 does not point to the slot, then control transfers from the test step 448 to a step 452 where a new slot is obtained for the data. Following the step 452 is a step 454 where the active one of the cache only virtual devices 404, 406 is made to point to the slot.

Following the step 454, or following the step 448 if the active one of the cache only virtual devices 404, 406 points to the slot, is a step 456 where flags are set. At the step 456, the RDF_WP flag (RDF write pending flag) is set to indicate that the slot needs to be transmitted to the remote storage device 26 using RDF. In addition, at the step 456, the IN CACHE flag is set to indicate that the slot needs to be destaged to the standard logical device 402. Note that, in some instances, if the active one of the cache only virtual devices 404, 406 already points to the slot (as determined at the step 448) it is possible that the RDF_WP and IN_CACHE flags were already set prior to execution of the step 456. However, setting the flags at the step 456 ensures that the flags are set properly no matter what the previous state.

Following the step 456 is a step 458 where an indirect flag in the track table that points to the slot is cleared, indicating that the relevant data is provided in the slot and not in a different slot indirectly pointed to. Following the step 458 is a step 462 where the data being written by the host 22 and the I-lA 28 is written to the slot. Following the step 462 is a step 464 where the slot is unlocked. Following step 464, processing is complete: If it is determined at the test step 446 that the inactive one of the cache only virtual devices 404, 406 points to the slot, then control transfers from the step 446 to a step 472, where a new slot is obtained. The new slot obtained at the step 472 may be used for the inactive one of the cache only virtual devices 404, 406 to effect the RJ)F transfer while the old slot may be associated with the active one of the cache only virtual devices 404, 406, as described below.

Following the step 472 is a step 474 where the data from the old slot is copied to the new slot that was obtained at the step 472. Following the step 474 is a step 476 where the indirect flag (discussed above) is set to indicate that the track table entry for the inactive one of the cache only virtual devices 404, 406 points to the old slot but that the data is in the new slot which is pointed to by the old slot. Thus, setting indirect flag at the step 476 affects the track table of the inactive one of the cache only virtual devices 404, 406 to cause the track table entry to indicate that the data is in the new slot.

Following the step 476 is a step 478 where the mirror bits for the records in the new slot are adjusted. Any local minor bits that were copied when the data was copied from the old slot to the new slot at the step 474 are cleared since the purpose of the new slot is to simply effect the RDF transfer for the inactive one of the cache only virtual devices. The old slot will be used to handle any local mirrors. Following the step 478 is the step 462 where the data is written to the slot. Following step 462 is the step 464 where the slot is unlocked. Following the step 464, processing is complete.

Referring to Figure 13, a flow chart 500 illustrates steps performed in connection with the local storage device 24 transmitting the chunk of data 54 to the remote storage device 26. The transmission essentially involves scanning the inactive one of the cache only virtual devices 404, 406 for tracks that have been written thereto during a previous iteration when the inactive one of the cache only virtual devices 404, 406 was active. In this embodiment, the DA's 35a-35c of the local storage device 24 scan the inactive one of the cache only virtual devices 404, 406 to copy the data for transmission to the remote storage device 26 by one or more of the RA's 30a-30c using the RDF protocol.

Processing begins at a first step 502 where the first track of the inactive one of the cache only virtual devices 404, 406 is pointed to in order to begin the process of iterating through all of the tracks. Following the first step 502 is a test step 504 where it is determined if the RDF_WP flag is set. As discussed elsewhere herein, the RDFWP flag is used to indicate that a slot (track) contains data that needs to be transmitted via the RDF link. The RDF WP flag being set indicates that at least some data for the slot (track) is to be transmitted using RDF. In an embodiment herein, the entire slot is not transmitted. Rather, only records within the slot that have the appropriate mirror bits set (indicating the records have changed) are transmitted to the remote storage device 26. However, in other embodiments, it may be possible to transmit the entire slot, provided that the remote storage device 26 only writes data corresponding to records having appropriate mirror bits set and ignores other data for the track, which may or may not be valid.

If it is determined at the test step 504 that the cache slot being processed has the RDFWP flag set, then control transfers from the step 504 to a test step 505, where it is determined if the slot contains the data or if the slot is an indirect slot that points to another slot that contains the relevant data. In some instances, a slot may not contain the data for the portion of the disk that corresponds to the slot. Instead, the slot may be an indirect slot that points to another slot that contains the data. If it is determined at the step 505 that the slot is an indirect slot, then control transfers from the step 505 to a step 506, where the data (from the slot pointed to by the indirect slot) is obtained.

Thus, if the slot is a direct slot, the data for being sent by RDF is stored in the slot while if the slot is an indirect slot, the data for being sent by RDF is in another slot pointed to by the indirect slot.

Following the step 506 or the step 505 if the slot is a direct slot is a step 507 where data being sent (directly or indirectly from the slot) is copied by one of the DA's 35a-35c to be sent from the local storage device 24 to the remote storage device 26 using the RDF protocol. Following the step 507 is a test step 508 where it is determined if the remote storage device 26 has acknowledged receipt of the data. If not, then control transfers from the step 508 back to the step 507 to resend the data. In other embodiments, different and more involved processing may used to send data and acknowledge receipt thereof. Such processing may include error reporting and alternative processing that is performed after a certain number of attempts to send the data have failed.

Once it is determined at the test step 508 that the data has been successfully sent, control passes from the step 508 to a step 512 to clear the RDF_\VP flag (since the data has been successfully sent via RDF). Following the step 512 is a step 514 where appropriate mirror flags are cleared to indicate that at least the RDF mirror (R2) no longer needs the data. In an embodiment herein, each record that is part of a slot (track) has individual mirror flags indicating which minors use the particular record.

The R2 device is one of the mirrors for each of the records and it is the flags corresponding to the R2 device that are cleared at the step 514.

Following the step 514 is a test step 516 which determines if any of the records of the track being processed have any other mirror flags set (for other mirror devices).

If not, then control passes from the step 516 to a step 518 where the slot is released (i.e., no longer being used). In some embodiments, unused slots are maintained in a pool of slots available for use. Note that if additional flags are still set for some of the records of the slot, it may mean that the records need to be destaged to the standard logical device 402 or are being used by some other minor (including another R2 device). Following the step 518, or following the step 516 if more minor flags are present, is a step 522 where the pointer that is used to iterate through each track entry of the inactive one of the cache only virtual devices 404, 406 is made to point to the next track. Following the step 522 is a test step 524 which determines if there are more tracks of the inactive one of the cache only virtual devices 404, 406 to be processed. If not, then processing is complete. Otherwise, control transfers back to the test step 504, discussed above. Note that the step 522 is also reached from the test step 504 if it is determined that the RDF WIP flag is not set for the track being processed.

Referring to Figure 14, a diagram 700 illustrates a host 702 coupled to a plurality of local storage devices 703-705. The diagram 700 also shows a plurality of remote storage devices 706-708. Although only three local storage devices 703-705 and three remote storage devices 706-708 are shown in the diagram 700, the system described herein may be expanded to use any number of local and remote storage devices.

Each of the local storage devices 703-705 is coupled to a corresponding one of the remote storage devices 706-708 so that, for example, the local storage device 703 is coupled to the remote storage device 706, the local storage device 704 is coupled to the remote storage device 707 and the local storage device 705 is coupled to the remote storage device 708. The local storage device is 703-705 and remote storage device is 706-708 may be coupled using the ordered writes mechanism described herein so that, for example, the local storage device 703 may be coupled to the remote storage device 706 using the ordered writes mechanism. As discussed elsewhere herein, the ordered writes mechanism allows data recovery using the remote storage device in instances where the local storage device and/or host stops working and/or loses data.

In some instances, the host 702 may run a single application that simultaneously uses more than one of the local storage devices 703-705. In such a case, the application may be configured to insure that application data is consistent (recoverable) at the local storage devices 703-705 if the host 702 were to cease working at any time and/or if one of the local storage devices 703-705 were to fail. However, since each of the ordered write connections between the local storage devices 703-705 and the remote storage devices 706-708 is asynchronous from the other connections, then there is no assurance that data for the application will be consistent (and thus recoverable) at the remote storage devices 706-708. That is, for example, even though the data connection between the local storage device 703 and the remote storage device 706 (a first local/remote pair) is consistent and the data connection between the local storage device 704 and the remote storage device 707 (a second local/remote pair) is consistent, it is not necessarily the case that the data on the remote storage devices 706, 707 is always consistent if there is no synchronization between the first and second local/remote pairs.

For applications on the host 702 that simultaneously use a piurality of local storage devices 703 -705, it is desirable to have the data be consistent and recoverable at the remote storage devices 706-708. This may be provided by a mechanism whereby the host 702 controls cycle switching at each of the local storage devices 703-705 so that the data from the application running on the host 702 is consistent and recoverable at the remote storage devices 706-708. This functionality is provided by a special application that runs on the host 702 that switches a plurality of the local storage devices 703 -705 into multi-box mode, as described in more detail below.

Referring to Figure 15, a table 730 has a plurality of entries 732-734. Each of the entries 732-734 correspond to a single local/remote pair of storage devices so that, for example, the entry 732 may correspond to pair of the local storage device 703 and the remote storage device 706, the entry 733 may correspond to pair of the local storage device 704 and the remote storage device 707 and the entry 734 may correspond to the pair of local storage device 705 and the remote storage device 708. Each of the entries 732-734 has a plurality of fields where a first field 736a-736c represents a serial number of the corresponding local storage device, a second field 738a-738c represents a session number used by the multi-box group, a third field 742a-742c represents the serial number of the corresponding remote storage device of the Iocallremote pair, and a fourth field 744a-744c represents the session number for the multi-box group. The table 730 is constructed and maintained by the host 702 in connection with operating in multi-box mode. In addition, the table 730 is propagated to each of the local storage devices and the remote storage devices that are part of the multi-box group. The table 730 may be used to facilitate recovery, as discussed in more detail below.

Different local/remote pairs may enter and exit multi-box mode independently in any sequence and at any time. The host 702 manages entry and exit of local storage device/remote storage device pairs into and out of multi-box mode. This is described in more detail below, Referring to Figure 16, a flowchart 750 illustrates steps perfonned by the host 702 in connection with entry or exit of a local/remote pair in to or out of multi-box mode. Processing begins at a first step 752 where multi-box mode operation is temporarily suspended. Temporarily suspending multi-box operation at the step 752 is useful to facilitate the changes that are made in connection with entry or exit of a remote/local pair in to or out of multi-box mode. Following the step 752, is a step 754 where a table like the table 730 of Figure 15 is modified to either add or delete an entry, as appropriate. Following the step 754 is a step 756 where the modified table is propagated to the local storage devices and remote storage devices of the multi-box group. Propagating the table at the step 756 facilitates recovery, as discussed in more detail elsewhere herein.

Following the step 756 is a step 758 where a message is sent to the affected local storage device to provide the change. The local storage device may configure itself to run in multi-box mode or not, as described in more detail elsewhere herein. As discussed in more detail below, a local storage device handling ordered writes operates differently depending upon whether it is operating as part of a multi-box group or not.

If the local storage device is being added to a multi-box group, the message sent at the step 758 indicates to the local storage device that it is being added to a multi-box group so that the local storage device should configure itself to run in multi-box mode.

Alternatively, if a local storage device is being removed from a multi-box group, the message sent at the step 758 indicates to the local storage device that it is being removed from the multi-box group so that the local storage device should configure itself to not run in multi-box mode.

Following step 758 is a test step 762 where it is determined if a local/remote pair is being added to the multi-box group (as opposed to being removed). If so, then control transfers from the test step 762 to a step 764 where tag values are sent to the local storage device that is being added. The tag values are provided with the data transmitted from the local storage device to the remote storage device in a manner similar to providing the sequence numbers with the data. The lag values are controlled by the host and set so that all of the local/remote pairs send data having the same tag value during the same cycle. Use of the tag values is discussed in more detail below.

Following the step 764, or following the step 762 if a new local/remote pair is not being added, is a step 766 where multi-box operation is resumed. Following the step 766, processing is complete.

Referring to Figure 17, a flow chart 780 illustrates steps performed in connection with the host managing cycle switching for multiple local/remote pairs running as a group in multi-box mode. As discussed elsewhere herein, multi-box mode involves having the host synchronize cycle switches for more than one remote/local pair to maintain data consistency among the remote storage devices. Cycle switching is coordinated by the host rather than being generated internally by the Jocal storage devices. This is discussed in more detail below.

Processing for the flow chart 780 begins at a test step 782 which determines if M seconds have passed. Just as with non-multi-box operation, cycle switches occur no sooner than every M seconds where M is a number chosen to optimize various performance parameters. As the number M is increased, the amount of overhead associated with switching decreases. However, increasing M also causes the amount of data that may be potentially lost in connection with a failure to also increase. In an embodiment herein, M is chosen to be thirty seconds, although, obviously other values for M may be used.

If it is determined at the test step 782 that M seconds have not passed, then control transfers back to the step 782 to continue waiting until M seconds have passed.

Once it is determined at the test step 782 that M seconds have passed, control transfers from the step 782 to a step 784 where the host queries all of the local storage devices in the multi-box group to determine if all of the local/remote pairs arc ready to switch.

The locallremote pairs being ready to switch is discussed in more detail hereinafter.

Following the step 784 is a test step 786 which determines if all of the loeallremote pairs are ready to switch. If not, control transfers back to the step 784 to resume the query. In an embodiment herein, it is only necessary to query local/remote pairs that were previously not ready to switch since, once a local/remote pair is ready to switch, the pair remains so until the switch occurs.

Once it is determined at the test step 786 that all of the locallremote pairs in the multi-box group are ready to switch, control transfers from the step 786 to a step 788 where an index variable, N, is set equal to one. The index variable N is used to iterate through all the local/remote pairs (i.e., all of the entries 732-734 of the table 730 of Figure 15). Following the step 788 is a test step 792 which determines if the index variable, N, is greater than the number of local/remote pairs in the multi-box group. If not, then control transfers from the step 792 to a step 794 where an open window is performed for the Nth local storage device of the Nth pair by the host sending a command (e.g., an appropriate system conrmand) to the Nth local storage device.

Opening the window for the Nth local storage device at the step 794 causes the Nth local storage device to suspend writes so that any write by a host that is not begun prior to opening the window at the step 794 will not be completed until the window is closed (described below). Not completing a write operation prevents a second dependant write from occurring prior to completion of the cycle switch. Any writes in progress that were begun before opening the window may complete prior to the window being closed.

Following the step 794 is a step 796 where a cycle switch is perfonned for the Nth local storage device. Performing the cycle switch at the step 796 involves sending a command from the host 702 to the Nth local storage device. Processing the command from the host by the Nth local storage device is discussed in more detail below. Part of the processing performed at the step 796 may include having the host provide new values for the tags that are assigned to the data. The tags are discussed in more detail elsewhere herein. In an alternative embodiment, the operations performed at the steps 794, 796 may be performed as a single integrated step 797, which is illustrated by the box drawn around the steps 794, 796.

Following the step 796 is a step 798 where the index variable, N, is incremented. Following step 798, control transfers back to the test step 792 to determine if the index variable, N, is greater than the number of local/remote pairs.

If it is determined at the test step 792 that the index variable, N, is greater than the number of local/remote pairs, then control transfers from the test step 792 to a step 802 where the index variable, N, is set equal to one. Following the step 802 is a test step 804 which determines if the index variable, N, is greater than the number of local/remote pairs. If not, then control transfers from the step 804 to a step 806 where the window for the Nth local storage device is closed. Closing the window of the step 806 is performed by the host sending a command to the Nth local storage device to cause the Nth local storage device to resume write operations. Thus, any writes in process that were suspended by opening the window at the step 794 may now be completed after execution of the step 806. Following the step 806, control transfers to a step 808 where the index variable, N, is incremented. Following the step 808, control transfers back to the test step 804 to determine if the index variable, N, is greater than the number of locallremote pairs. If so, then control transfers from the test step 804 back to the step 782 to begin processing for the next cycle switch.

Referring to Figure 18, a flow chart 830 illustrates steps performed by a local storage device in connection with cycle switching. The flow chart 830 of Figurel8 replaces the flow chart 350 of Figure 10 in instances where the local storage device supports both multi-box mode and non-multi-box mode. That is, the flow chart 830 shows steps performed like those of the flow chart 350 of Figure 10 to support non-multi-box mode and, in addition, includes steps for supporting multi-box mode.

Processing begins at a first test step 832 which determines if the local storage device is operating in multi-box mode. Note that the flow chart 750 of Figure 16 shows the step 758 where the host sends a message to the local storage device. The message sent at the step 758 indicates to the local storage device whether the local storage device is in multi-box mode or not. Upon receipt of the message sent by the host at the step 758, the local storage device sets an internal variable to indicate whether the local storage device is operating in multi-box mode or not. The internal variable may be examined at the test step 832.

If it is determined at the test step 832 that the local storage device is not in mtilti-box mode, then control transfers from the test step 832 to a step 834 to wait M seconds for the cycle switch. If the local storage device is not operating in multi-box mode, then the local storage device controls its own cycle switching and thus executes the step 834 to wait M seconds before initiating the next cycle switch.

Following the step 834, or foilowing the step 832 if the local storage device is in multi-box mode, is a test step 836 which determines if all of the HA's of the local storage device have set a bit indicating that the HA's have completed all of the 110's for a previous sequence number. When the sequence number changes, each of the HA's notices the change and sets a bit indicating that a11 110's of the previous sequence number are completed. For example, if the sequence number changes from N- I to N, an HA will set the bit when the HA has completed all I/O's for sequence number N-i.

Note that, in some instances, a single 1/0 for an HA may take a long time and may still be in progress even after the sequence number has changed. Note also that, for some systems, a different mechanism may be used to determine if all HA's have completed their N-i I/O's. The different mechanism may include examining device tables. Once it is determined at the test step 836 that all HA's have set the appropriate bit, control transfers from the test step 836 to a step 888 which determines if the inactive chunk for the local storage device is empty. Once it is determined at the test step 888 that the inactive chunk is empty, control transfers from the step 888 to a step 899, where copying of data from the local storage device to the remote storage device is suspended.

It is useful to suspend copying data until the sequence number switch is complete.

Following the step 899 is a test step 892 to determine if the local storage device is in multi-box mode. If it is determined at the test step 892 that the local storage device is in multi-box mode, then control transfers from the test step 892 to a test step 894 to determine if theactive chunk of the corresponding remote storage device is empty. As discussed in more detail below, the remote storage device sends a message to the local storage device once it has emptied its active chunk. In response to the message, the local storage device sets an internal variable that is examined at the test step 894.

Once it is determined at the test step 894 that the active chunk of the remote storage device is empty, control transfers from the test step 894 to a step 896 where an internal variable is set on a local storage device indicating that the local storage device is ready to switch cycles. As discussed above in connection with the flow chart 780 of Figure 17, the host queries each of the local storage devices to determine if each of the local storage devices are ready to switch. In response to the query provided by the host, the local storage device examines the internal variable set at the step 896 and returns the result to the host.

Following step 896 is a test step 898 where the local storage device waits to receive the command from the host to perform the cycle switch. As discussed above in connection with the flow chart 780 of Figure 17, the host provides a command to switch cycles to the local storage device when the local storage device is operating in multi-box mode. Thus, the local storage device waits for the command at the step 898, which is only reached when the local storage device is operating in multi-box mode.

Once the local storage device has received the switch command from the host, control transfers from the step 898 to a step 902 to send a commit message to the remote storage device. Note that the step 902 is also reached from the test step 892 if it is determined at the step test 892 that the local storage device is not in multi-box mode.

At the step 902, the local storage device sends a commit message to the remote storage device. In response to receiving a commit message for a particular sequence number, the remote storage device will begin restoring the data corresponding to the sequence number, as discussed above.

Following the step 902 is a step 906 where the sequence number is incremented and a new value for the tag (from the host) is stored. The sequence number is as S discussed above. The tag is the tag provided to the local storage device at the step 764 and at the step 796, as discussed above. The tag is used to facilitate data recovery, as discussed elsewhere herein.

Following the step 906 is a step 907 where completion of the cycle switch is confirmed from the local storage device to the host by sending a message from the local storage device to the host. In some embodiments, it is possible to condition performing the step 907 on whether the local storage device is in multi-box mode or not, since, if the local storage device is not in multi-box mode, the host is not necessarily interested in when cycle switches occur.

Following the step 907 is a step 908 where the bits for the HA's that are used in the test step 836 are all cleared so that the bits may be set again in connection with the increment of the sequence number. Following the step 908 is a test step 912 which determines if the remote storage device has acknowledged the commit message Note that if the local/remote pair is operating in multi-box mode and the remote storage device active chunk was determined to be empty at the step 894, then the remote storage device should acknowledge the commit message nearly immediately since the remote storage device will be ready for the cycle switch immediately because the active chunk thereof is already empty.

Once it is determined at the test step 912 that the commit message has been acknowledged by the remote storage device, control transfers from the step 912 to a step 914 where the suspension of copying, which was provided at the step 899, is cleared so that copying from the local storage device to the remote storage device may resume. Following the step 914, processing is complete.

Referring to Figure 19, a flow chart 940 illustrates steps performed in connection with RA's scanning the inactive buffers to transmit RDF data from the local storage device to the remote storage device. The flow chart 940 of Figure 19 is similar to the flow chart 200 of Figure 6 and similar steps arc given the same reference number. However, the flow chart 940 includes two additional steps 942, 944 which are not found in the flow chart 200 of Figure 6. The additional steps 942, 944 are used to facilitate multi-box processing. After data has been sent at the step 212, control transfers from the step 212 to a test step 942 which determines if the data being sent is the last data in the inactive chunk of the local storage device. If not, then control transfers from the step 942 to the step 214 and processing continues as discussed above in connection with the flow chart 200 of Figure 6. Otherwise, if it is determined at the test step 942 that the data being sent is the last data of the chunk, then control transfers from the step 942 to the step 944 to send a special message from the local storage device to the remote storage device indicating that the last data has been sent.

Following the step 944, control transfers to the step 214 and processing continues as discussed above in connection with the flow chart 200 of Figure 6. In some embodiments, the steps 942, 944 may be performed by a separate process (and/or separate hardware device) that is different from the process andlor hardware device that transfers the data.

Referring to Figure 20, a flow chart 950 illustrates steps performed in connection with RA's scanning the inactive buffers to transmit RDF data from the local storage device to the remote storage device. The flow chart 950 of Figure 20 is similar to the flow chart 500 of Figure 13 and similar steps are given the same reference number. However, the flow chart 950 includes an additional step 952, which is not found in the flow chart 500 of Figure 13. The additional steps 952 is used to facilitate multi-box processing and is like the additional step 944 of the flowchart 940 of Figure 19. After it is determined at the test step 524 that no more slots remain to be sent from the local storage device to the remote storage device, control transfers from the step 524 to the step 952 to send a special message from the local storage device to the remote storage device indicating that the last data for the chunk has been sent.

Following the step 952, processing is complete.

Referring to Figure 21, a flow chart 960 illustrates steps performed at the remote storage device in connection with providing an indication that the active chunk of the remote storage device is empty. The flow chart 960 is like the flow chart 300 of Figure 9 except that the flow chart 960 shows a new step 962 that is performed after the active chunk of the remote storage device has been restored. At the step 962, the remote storage device sends a message to the local storage device indicating that the active chunk of the remote storage device is empty. Upon receipt of the message sent at the step 962, the local storage device sets an internal variable indicating that the inactive buffer of the remote storage device is empty. The local variable is examined in connection with the test step 894 of the flow chart 830 of Figure 18, discussed above.

Referring to Figure 22, a diagram 980 illustrates the host 702, local storage devices 703-705 and remote storage devices 706-708, that are shown in the diagram 700 of Figure 14. The Diagram 980 also includes a first alternative host 982 that is coupled to the host 702 and the local storage devices 703-705. The diagram 980 also includes a second alternative host 984 that is coupled to the remote storage devices 706-708. The alternative hosts 982, 984 may be used for data recovery, as described in more detail below.

When recovery of data at the remote site is necessary, the recovery may be performed by the host 702 or, by the host 982 provided that the links between the local storage devices 703-705 and the remote storage devices 706-708 are still operational.

If the links are not operational, then data recovery may be performed by the second alternative host 984 that is coupled to the remote storage devices 706-708. The second alternative host 984 may be provided in the same location as one or more of the remote storage devices 706-708. Alternatively, the second alternative host 984 may be remote from all of the remote storage devices 706-708. The table 730 that is propagated throughout the system is accessed in connection with data recovery to determine the members of the multi-box group.

Referring to Figure 23, a flow chart 1000 illustrates steps performed by each of the remote storage devices 706-708 in connection with the data recovery operation.

The steps of the flowchart 1000 may be executed by each of the remote storage devices 706-708 upon receipt of a signal or a message indicating that data recovery is necessary. In some embodiments, it may be possible for a remote storage device to automatically sense that data recovery is necessary using, for example, conventional criteria such as length of time since last write.

Processing begins at a first step 1002 where the remote storage device finishes restoring the active chunk in a manner discussed elsewhere herein. Following the step 1002 is a test step 1004 which determines if the inactive chunk of the remote storage device is complete (i.e., all of the data has been written thereto). Note that a remote storage device may determine if the inactive chunk is complete using the message sent by the local storage device at the steps 944, 952, discussed above. That is, if the local storage device has sent the message at the step 944 or the step 952, then the remote storage device may use receipt of that message to confirm that the inactive chunk is complete.

If it is determined at the test step 1004 that the inactive chunk of the remote storage device is not complete, then control transfers from the test step 1004 to a step 1006 where the data from the inactive chunk is discarded. No data recovery is performed using incomplete inactive chunks since the data therein may be inconsistent with the corresponding active chunks. Accordingly, data recovery is performed using active chunks and, in some cases, inactive chunks that are complete. Following the step 1006, processing is complete.

If it is determined at the test step 1004 that the inactive chunk is complete, then control transfers from the step 1004 to the step 1008 where the remote storage device waits for intervention by the host. if an inactive chunk, one of the hosts 702, 982, 984, as appropriate, needs to examine the state of all of the remote storage devices in the multi-box group to determine how to perform the recovery. This is discussed in more detail below.

Following step 1008 is a test step 1012 where it is determined if the host has provided a command to all storage device to discard the inactive chunk. If so, then control transfers from the step 1012 to the step 1006 to discard the inactive chunk.

Following the step 1006, processing is complete.

If it is determined at the test step 1002 that the host has provided a command to restore the complete inactive chunk, then control transfers from the step 1012 to a step 1014 where the inactive chunk is restored to the remote storage device. Restoring the inactive chunk in the remote storage device involves making the inactive chunk an active chunk and then writing the active chunk to the disk as described e]sewhere herein. Following the step 1014, processing is complete.

Referring to Figure 24, a flow chart 1030 illustrates steps performed in connection with one of the hosts 702, 982, 984 determining whether to discard or restore each of the inactive chunks of each of the remote storage devices. The one of the hosts 702, 982, 984 that is performing the restoration communicates with the remote storage devices 706-708 to provide commands thereto and to receive information therefrom using the tags that are assigned by the host as discussed elsewhere herein.

Processing begins at a first step 1032 where it is determined f any of the remote storage devices have a complete inactive chunk. If not, then there is no further processing to be performed and, as discussed above, the remote storage devices will discard the incomplete chunks on their own without host intervention. Otherwise, control transfers from the test step 1032 to a test step 1034 where the host determines if all of the remote storage devices have complete inactive chunks. If so, then control transfers from the test step 1034 to a test step 1036 where it is determined if all of the complete inactive chunks of all of the remote storage devices have the same tag number. As discussed elsewhere herein, tags are assigned by the host and used by the system to identify data in a manner similar to the sequence number except that tags are controlled by the host to have the same value for the same cycle.

If it is detennined at the test step 1036 that all of the remote storage devices have the same tag for the inactive chunks, then control transfers from the step 1036 to a step 1038 where all of the inactive chunks are restored. Performing the step 1038 ensures that all of the remote storage devices have data from the same cycle. Following the step 1038, processing is complete.

If it is determined at the test step 1034 that all of the inactive chunks are not complete, or if it is determined that at the step 1036 that all of the complete inactive chunks do not have the same tag, then control transfers to a step 1042 where the host provides a command to the remote storage devices to restore the complete inactive chunks having the lower tag number. For purposes of explanation, it is assumed that the tag numbers are incremented so that a lower tag number represents older data. By way of example, if a first remote storage device had a complete inactive chunk with a tag value of three and a second remote storage device had a complete inactive chunk with a tag value of four, the step 1042 would cause the first remote storage device (but not the second) to restore its inactive chunk. Following the step 1042 is a step 1044 where the host provides commands to the remote storage devices to discard the complete inactive buffers having a higher tag number (e.g, the second remote storage device in the previous example). Following step 1044, processing is complete.

Following execution of the step 1044, each of the remote storage devices contains data associated with the same tag value as data for the other ones of the remote storage devices. Accordingly, the recovered data on the remote storage devices 706- 708 should be consistent.

Referring to Figure 25, a diagram 1120 illustrates an embodiment where a storage device 1124, which is like the storage device 24 andlor the storage device 26, includes a plurality of directors 11 52a-11 52c that are coupled to a memory 1126, like the memory 37 and/or the memory 38 of the storage devices 24, 26. Each of the directors 1152a-l152c represents an HA, DA, and/or RA like the HA 28, the DA's 35a- 35c, 36a-36c, and/or the RA's 30a-30c, 32a-32c of the storage devices. In an embodiment disclosed herein, there may be up to sixteen directors coupled to the memory 1126. Of course, for other embodiments, there maybe a higher or lower maximum number of directors that may be used.

The diagram 1120 also shows an optional communication module (CM) 1154 that provides an alternative communication path between the directors 11 52a-11 52c.

Each of the directors 1 152a-1 152c may be coupled to the CM 1154 so that any one of the directors 11 52a-11 52c may send a message andlor data to any other one of the directors I 152a-1 I 52c without needing to go through the memory 1126. The CM 1154 may be implemented using conventional MlJXlrouter technology where a sending one of the directors 1152a-1 152c provides an appropriate address to cause a message andior data to be received by an intended receiving one of the directors 11 52a-11 52c. In addition, a sending one of the directors 1152a-1152c may be able to broadcast a message to all of the other directors 1152a-1152c at the same time.

Refer to Figure 26, a diagram 1200 illustrates a system that includes a source group 1202, a local destination 1204, and a remote destination 1206. The source group 1202 communicates with both the local destination 1204 and the remote destination 1206. The source group 1202 may represent a single storage device, a plurality of storage devices, a single or plurality of storage devices with a single host, or a single or plurality of storage devices with multiple hosts. Data is generated at the source group 1202 and stored at the source group 1202 and transmitted to the local destination 1204 and the remote destination 1206. In some embodiments, the local group may operate in some ways as discussed above in connection with the embodiment described above in connection with Figure 14.

In an embodiment herein, the local destination 1204 represents a single or plurality of storage devices that are in relatively close proximity to the source group 1202 to allow for synchronous data mirroring from the source group 1202 to the local destination 1204. In an embodiment herein, the local destination 1204 is located in the same building, at the same facility, andlor at the same corporate location as the source group 1202. Thus, for example, the local destination 1204 may represent a backup storage device (or plurality of storage devices) managed by the same group of individuals that manages the source group 1202.

The remote destination 1206 represents a storage device andlor a plurality of storage devices at a location that is geographically distant from the source group 1202.

For example, the remote destination 1206 may represent a storage device or group of storage devices on the west coast of the United States while the source group 1202 is located on the east coast of the United States. Because of the relatively large geographic distance between the source group 1202 and the remote destination 1206, it may be impractical to use a synchronous or semi-synchronous data transfer mode to mirror data on the remote destination 1206 from the source group 1202. That is, because of the long round trip time from the source group 1202 to the remote destination 1206 and back to the source group 1202, it may not be feasible to use synchronous or semi-synchronous data mirroring which provides for acknowledgment of data at the remote destination 1206 prior to acknowledging a write to a host of the local group 1202. In such case, it may be possible to use ordered writes between the source group 1202 and the remote destination 1206 so that the remote destination 1206 is a consistent, although somewhat time delayed, mirror of the source group 1202.

A communication path 1208 may also exist between the local destination 1204 and the remote destination 1206. As long as the source group 1202 is operational, the communication path 1208 may not be used. J-Iowever, in instances where the source group 1202 becomes non-operational and/or in instances where the communication links between the source group 1202 and one or both of the local destination 1204 and the remote destination 1206 become non-operational, then the communication path 1208 may be used to synchronize the data between the local destination 1204 and the remote destination 1206, as described in more detail elsewhere herein. In addition, a host may be provided to one of the local destination 1204 or the remote destination 1206 to resume operation of the system after synchronization, as described in more detail elsewhere herein. Note that if the operation is resumed with a host coupled to the local destination 1204, then the communication link 1208 may be used so that the remote destination 1206 is a mirror of the local destination 1204. Conversely, if operation is resumed by providing a host to the remote destination 1206, then the communication link 1208 may be used so that the local destination 1204 is a mirror of the remote destination 1206.

Referring to Figure 27, a flow chart 1230 illustrates steps performed in connection with initializing parameters at the local destination 1204 to provide the recovery mechanism discussed herein. The recovery mechanism described herein uses the Symmetrix Differential Data Facility (SDDF), which allows for a plurality sessions, each having a bitmap associated therewith with bits that are set to one whenever there is a write to a corresponding data location during a particular time period. If no write occurs to a particular location, the corresponding bit remains cleared (i.e., remains zero). In an embodiment herein, each bit may correspond to a track of data. However, for other embodiments, it is possible to have each bit correspond to larger or smaller increments of data and it is also possible that different bits andlor different sets of bits correspond to different amounts of data.

Operations associated with an SDDF session include creating an SDDF session, activating an SDDF session, clearing bits of the bitmap for an SDDF session, deactivating an SDDF session, and terminating an SDDF session. Creating an SDDF session, clearing the bits thereof, and terminating an SDDF session are fairly self-explanatory. Activating an SDDF session causes a bit of the SDDF session bitmap to be set whenever a corresponding track (or other appropriate data increment) is written.

Deactivating an SDDF session suspends the setting of bits. The SDDF mechanism discussed herein may be implemented using the description provided in U.S. Patent No. 6,366,986, which is incorporated by reference herein.

Processing for the flow chart 1230 begins at a first step 1232 where a first SDDF session, SDDF 1, is created. In an embodiment described herein, creation of an SDDF session does not cause automatic activation of the session. Following step 1232 is a step 1234 where the bits of the bitmap of the SDDF session created at the step 1232 are cleared. Following step 1234 is a step 1236 where a second SDDF session, SDDF2, is created. Following step 1236 is a step 1238 where the bits of the bitmap of the SDDF session created at the step 1236 are cleared.

Following the step 1238 is a step 1242 where a state is initialized. The state initialized at the step 1242 may be used to determine which of the SDDF sessions, SDDF1 or SDDF 2, will be activated. As described in more detail elsewhere herein, there may be two possible states and the state set at the step 1242 may be toggled to cause the SDDF1 session and the SDDF_2 session to be alternatively activated. In other embodiments, a token or some other type of variable may be used to indicate the selection of either SDDF_1 or SDDF_2. Following the step 1242 is a step 1244 where SDDF_1 is activated. Activating SDDF1 at the step 1244 causes the bits of the bit map of the SDDF_1 session to be set whenever a track (or other data increment) of the local destination 1204 is modified.

The SDDFJ and SDDF_2 sessions are used by the local destination 1204 to keep track of the active and inactive buffers used by the source group 1202 in connection with ordered writes by the source group 1202 to the remote destination 1206. As discussed in more detail elsewhere herein, each time the source group 1202 makes a cycle switch in connection with ordered writes from the source group 1202 to the remote destination 1206, the source group 1202 sends a message to the local destination 1204 indicating that a cycle switch has been performed so that the local destination 1204 may toggle the state (initialized in the step 1242, discussed above).

Use of the cycle switch information by the local destination 1204 is discussed in more detail elsewhere herein.

Referring to Figure 28A, a flow chart 1250 illustrates steps performed by the local destination 1204 in connection with receiving an I/O from the source group 1202 during normal (i.e., non-failure) operation. Processing begins at a first step 1252 where the 110 is received by the local destination 1204. Following the step 1252 is a test step 1254 which determines if the 110 received from the source group 1202 indicates that the local group 1202 is ready to switch in connection with ordered writes between the local group 1202 and the remote destination 1206. The local group 1202 being rcady to switch is described in more detail elsewhere herein.

If the received data indicates that the local group 1202 is ready to switch, then control transfers from the step 1254 to a step 1256, where it is determined if the inactive one of the SDDF sessions (SDDF1 or SDDF 2) is clear. In some embodiments, the SDDF sessions may be cleared at the step 1256. Tn other instances, the amount of time needed to clear an SDDF session at the step 1256 would be unacceptable, in which case more than two SDDF sessions may be used for SDDFj and SDDF_2 and may be rotated so that an SDDF session that is about to be activated is always cleared asynchronously. In any event, the processing performed at the step 1256 relates to clearing the inactive one ofSDDF1 and SDDF2 so that, after performing the step 1256, the inactive session is clear.

Following the step 1256 is a step 1258 where the inactive one of the SDDF sessions is activated so that both SDDF_1 and SDDF2 are activated after performing the processing at the step 1258. Thus, subsequent writes reflected in the bitmaps for both SDDF_1 and SDDF_2. Following the step 1258, processing is complete.

If it is determined at the step 1254 that the received data does not correspond to a ready to switch signal, then control transfers from the step 1254 to a test step 1262 where it is determined if the received data corresponds to a cycle switch being performed. If so, then control transfers from the step 1262 to a step 1264 where the state, initialized at the step 1242 of the flow chart 1230 of Figure 27, is toggled. As discussed elsewhere herein, the state is used to determine which one of SDDF_1 and SDDF2 will be activated and deactivated. Following the step 1264 is a step 1266, rhere one of the SDDF sessions, SDDF_l or SDDF_2, is deactivated, depending on the particular value of the state set at the step 1264. Note that even though an SDDF session is deactivated at the step 1266, that SDDF session is not cleared until the next ready to switch signal is received. Of course, if more than two SDDF sessions are used for SDDF1 and SDDF_2, as discussed above, then the SDDF session deactivated at the step 1266 may be maintained while another SDDF session is cleared to prepare for being activated at the step 1258, discussed above.

If it is determined at the step 1262 that the received data does not correspond to a cycle switch, then control transfers from the test step 1262 to a step 1268 where the 1/0 is performed. For example, if the I/O is a write operation, then, at the step 1268, data is written to the storage area of the local destination 1204. Following step 1268 is a step 1272 where it is determined if the 110 operation is a write operation. If not (e.g., the I/O operation is a read operation), then processing is complete. Otherwise, control transfers from the step 1272 to a step 1274 where a bit is set in the appropriate one of the SDDF sessions, SDDF 1, SDDF 2, or both depending upon which one of the SDDF sessions is activated. Following step 1274, processing is complete.

In some instances, it may not be desirable to wait to clear an SDDF bitmap just prior to pointing the same SDDF bitmap. In those cases, it may be useful to have more than two SDDF bitmaps where two at a time are used like SDDF_1 and SDDF_2 while the remainder of the SDDF bitmaps are alreadyclear and waiting to be used or are being cleared using a background process. For example, using three bitmaps SDDF_A, SDDFB, and SDDF C, SDDF I may correspond to SDDF_A while SDDF2 may correspond to SDDF_C. In such a case, SDDF_B may be cleared while processing is being performed on SDDF_A and SDDF_C. When the cycle switches, SDDFB (which is already clear) may be used while SDDF_C is cleared using a background process that may run even after the cycle switch is complete and new data is being logged to SDDFB.

Referring to Figure 28B, a flow chart 1280 illustrates an alternative embodiment that uses a plurality of SDDF bitmaps, SDDF[0], SDDF[1],. . .SDDF[NMAPS-l], where NMAPS is the number of SDDF maps. Tn an embodiment herein, NMAPS is greater than two (e.g, three). Many of the step of the flow chart 1280 are like steps of the flow chart 1250 of Figure 28A.

Processing begins at a first step 1282 where the 110 is received by the local destination 1204. Following the step 1282 is a test step 1283 which determines if the 1/0 received from the source group 1202 indicates that the local group 1202 is ready to * switch in connection with ordered writes between the local group 1202 and the remote destination 1206. The local group 1202 being ready to switch is described in LflOre detail elsewhere herein.

If the received data indicates that the local group 1202 is ready to switch, then control transfers from the step 1283 to a step 1284 where an index, K, is incremented and the result thereof is set to modulo NMAPS. Following the step 1284 is a step 1285, where it is confirmed that SDDF[K] is clear. Following the step 1285 is a step 1286 where SDDF[K] is activated so that both SDDF[K] and SDDF[K-1] are activated after -performing the processing at the step 1286. Thus, subsequent writes reflected in the bitmaps for both SDDF[KJ and SDDF[K-1J. Following the step 1286, processing is complete. Note that, if K is zero, then SDDF[K-1] actually refers to SDDF[NMAPS-1].

If it is determined at the step 1283 that the received data does not correspond to a ready to switch signal, then control transfers from the step 1283 to a test step 1287, where it is determined if the received data corresponds to a cycle switch. If it is determined at the step 1287 that the received data corresponds to a cycle switch (see discussion above in connection with the flow chart 1250 of Figure 28A), then control transfers from the step 1287 to a step 1288 where the state (discussed above) is toggled.

Following the step 1288 is a step 1289 where a variable J is set equal to (K-2) modulo NMAPS. Since K is an index variable used to keep track of the most recently activated SDDF bitmap, setting I at the step 1289 causes J to point to the third most recently activated SDDF bitmap. Following the step 1289 is a step 1292 where a process is started to clear the SDDF[J] bitmap. As discussed elsewhere herein, it is not necessary for the process begun at the step 1292 to be completed in order to complete the cycle -switch and to begin accumulating new data.

Following the step 1292 is a step 1294 where a variable J is set equal to (K-i) modulo NMAPS. Since K is an index variable used to keep track of the most recently activated SDDF bitmap, setting J at the step 1294 causes J to point to the second most recently activated SDDF bitmap. Following the step 1294 is a step 1296 where SDDF[J] is deactivated. 1-lowever, even though SDDF[J] is deactivated at the step 1296, the data is maintained for restoration purposes until the next cycle switch.

Following the step 1296, processing is complete.

Referring to Figure 28C, a flow chart 1300 illustrates yet another embodiment for processing related to the local destination 1204 receiving an 1/0 from the source group 1202 during normal (i.e., non-failure) operation. Processing begins at a first step 1302 where the 110 is received by the local destination 1204. Following the step 1302 is a test step 1304 where it is determined if the received data corresponds to a cycle switch being perfonned. If so, then control transfers from the step 1304 to a test step 1306 where it is determined if two or more cycle switches have occurred since the last time the state was toggled. If not, then processing is complete. Otherwise, conl.rol transfers from the step 1306 to a step 1307 where it is determined if the currently maclive SDDF session, SDDF X, is clear. If so, then control transfers from the step 1307 to a step 1308 where the state, initialized at the step 1242 of the flow chart 1230 of Figure 27, is toggled. As discussed elsewhere herein, the state is used to determine which one of SDDF I and SDDF_2 will be activated and deactivated.

Following the step 1308 is a step 1309 where one of the SDDF sessions, SDDFI or SDDF_2, as indicated by the state, is activated. Following the step 1309 is a step 1312 where the other one of the SDDF sessions is deactivated. Following the step 1312, processing is complete.

If it is determined at the test step 1307 that SDDF X is not clear, then control transfers from the step 1307 to a step 1313, where it is determined if clearing SDDF_X has already been started (i.e., in a previous iteration). If so, then processing is complete. Otherwise, control transfers from the step 1313 to a step 1314, where a process to clear SDDF_X is begun. Following the step 1314, processing is complete.

If it is determined at the step 1304 that the received data does not correspond to a cycle switch, then control transfers from the test step 1304 to a step 1316 where the I/O is performed. For example, if the I/O is a write operation, then, at the step 1316 data is written to the storage area of the local destination 1204. Following step 1316 is a step 1317 where it is determined if the I/O operation is a write operation. If not (e.g., the 110 operation is a read operation), then processing is complete. Otherwise, control transfers from the step 1317 to a step 1318 where a bit is set in the appropriate one of the SDDF sessions, SDDFI or SDDF_2, (SDDF X or SDDF_Y) depending upon which one of the SDDF sessions is activated. Following step 1318, processing is complete.

Referring to Figure 29, a flow chart 1320 illustrates steps performed in connection with collecting recovery data at the remote destination 1206. Unlike the local destination 1204, which is always collecting recovery data during steady state operation, the remote destination 1206 may not collect recovery data unless and until it receives a signal to do so. In an embodiment herein, the source group 1202 may provide such a signal to the remote destination 1206 when, for example, the source group 1202 detects that the link(s) between the source group 1202 and the local destination 1204 are not working andlor when the source group 1202 detects that the local destination 1204 is not appropriately receiving or processing data. In other embodiments or in other situations, the remote destination 1206 may receive an appropriate signal from a location or element other than the source group 1202.

Processing begins at a first step 1322 where a third SDDF session, SDDF3 is created. Following the step 1322 is a step 1324 where the bitmap of the SDDF session created at the step 1322 is cleared. Following step 1324 is a step 1326 where a token value (described in more detail elsewhere herein) is set to zero. Following the step 1326, processing is complete.

Referring to Figure 30, a flow chart 1330 illustrates steps performed by the remote destination 1206 in connection with collection of recovery data. Processing begins at a first step 1331 where the remote destination 1206 waits for a failure message from the source group 1202 or from some other source, as appropriate. Once a failure message has been received, control transfers from the step 133 ito a step 1332 where SDDF3 session is activated to begin collecting data iegarding the tracks (or other appropriate data increments) of the remote destination 1206 to which a write has S been performed. Note, however, that SDDF_3 reflects writes that have been committed (i.e.,. are one behind the current cycle being received).

Following the step 1332 is a step 1333 where a token value (described below) is initialized to zero. Following the step 1333 is a step 1334 where the remote destination 1206 receives an 110 from the source group 1202. Note that the I/O received at the step 1334 may represent ordered writes data that the local group 1202 sends to the remote destination 1206 in non-failure mode.

Following the step 1334 is a test step 1336 which determines if the I/O received from the source group 1202 at the step 1334 indicates a cycle switch by the source group 1202. If not (i.e., the data is conventional ordered writes data to be written to the remote destination 1206 or some other type of data), then control transfers from the test step 1336 back to the step 1334 to receive the next I/O.

If it is determined at the test step 1336 that the data from the source group 1202 indicates a cycle switch, then control transfers from the test step 1336 to a step 1338 to increment the token, which keeps track of the number of cycle switch since beginning collection of recovery data. Following the step 1338 is a step 1342 where the bitmap of the SDDF3 is set to reflect data of the conimitted cycle that had been written. Note that the processing at the step 1342 may be performed automatically in connection with operation of the activated SDDF sessions, in which case in may not be necessary to make the step 1342 part of the recovery process shown if Figure 30.

Note that the committed cycle is the cycle used to collect data prior to the cycle switch. Following the step 1342 is a test step 1344 which determines if processing is complete (i.e., collection of error recovery data is complete). Processing may be complete if the error situation (e.g., failed link from the local group 1202 to the local destination 1204) has been rectified or, for example, if the local destination 1204 and the remote destination 1206 are being synchronized (discussed below). If it is determined at the step 1344 that processing is not complete, then control transfers from the test step 1344 back to the step 1334 to receive another 110. Otherwise, control transfers from the step 1344 back to the step 1331 to wait for a new failure message.

Note that, in some embodiments, processing being complete at the step 1344 may also cause certain recovery, parameters to be reset, as discussed in more detail below.

Referring to Figure 31, a flow chart 1360 indicates steps performed in coimection with synchronizing the local destination 1204 and the remote destination 1206. As discussed above, the local destination 1204 and the remote destination 1206 may be synchronized by transferring data therebetween using the conmiun.ication link 1208. After synchronization, the system may be restarted at the local destination 1204 or at the remote destination 1206 using the other as a mirror.

In connection with the synchronization, it may be useful to decide which of the local destination 1204 and remote destination 1206 has the latest (i.e., the most up-to-date) data so that the data is transferred from the one of the remote destination 1206 and local destination 1204 that has the most up-to-date data to the other.

Processing for the flow chart 1360 begins at a first step 1362, where the links are dropped between the source group 1202 and the local destination 1204 and the links are dropped between the source group 1202 and the remote destination 1206. Dropping the links at the step 1362 helps ensure that the synchronization process will not be corrupted. Following the step 1362 is a step 1364 where the SDDF maps, created while recovery information was being collected, is preserved. Following the step 1364 is a step 1366 where ordered writes between the source group 1202 and the local destination 1204 is terminated, for reasons similar to dropping the links at the step Following the step 1366 is a test step 1368 where it is determined if the token value (discussed above) is greater than a predetermined value N. Note that the token value indicates the number of cycle switches that ha'e occurred since collection of error recovery data began at the remote destination 1206. If the link between the source group 1202 and the local destination 1204 has not been working and the remote destination 1206 has begun collection of rccovcry data, then the remote destination 1206 may contain more up-to-date data than the local destination 1204. This will be determined by looking at the value of the token, which indicates the number of cycle switches that have occurred since the remote destination 1206 received a signal to begin collecting recovery data. Thus, if it is determined at the test step 1368 that the token is greater than some prc-dctcrmined value N (e.g., two), then control transfers from the test step 1368 to a step 1371, where the bitmaps for all of the SDDF sessions (SDDF_I, SDI)F_2, and Si)DF_3) are ORed (using an inclusive OR) to determine the tracks (or other data aniounts) of the remote destination 1206 and possibly of the local destination 1204 that correspond to data for the active and inactive buffers sent or in transit between the source group 1202 and the remote destination 1206 prior to failure of the source group as well as possible data that may be different on the local destination 1204.

Following the step 1371 is a step 1372 where the remote destination 1206 sends data from the tracks corresponding to the set bits of the bitmap that was the result or ORing the three bitmaps for SDDF 1, SDDF 2, and SDDF_3. The data from these tracks may be copied to the local destination 1204 so that the remote destination 1206 and the local destination 1204 may be synchronized. Following the step 1372, processing is complete. In an embodiment herein, N may be set to be no lower than two. Also, note that it may be possible to resume operation with a host coupled to an appropriate one of the local destination 1204 or the remote destination 1206 prior to completion of the copies initiated at the step 1376 or at the step 1372.

If it is determined at the test step 1368 that the token does not have a value greater than N (e.g., the token is zero), then control transfers from the test step) 368 to a step 1374 where the bitmaps for all of the SDDF sessions (SDDF_1, SDDF_2, and, if it exists, SDDF_3) are ORed (using an inclusive OR) to determine the tracks (or other data amounts) of the local destination 1204 that correspond to data for the active and inactive buffers sent or in transit between the source group 1202 and the remote destination 1206 prior to failure of the source group 1202. Following the step 1374 is a step 1376 where the data corresponding to the ORing of the bitmaps is sent from the local destination 1204 to the remote destination 1206 via the communication link 1208.

Once the data is sent from the local destination 1204 to the remote destination 1206, then the local destination 1204 and the remote destination 1206 will be synchronized.

Following the step 1376, processing is complete.

The step 1372, 1374, 1376 may be accomplished using any number of appropriate techniques. For example, a background copy may be initiated to copy the data using bits indicated by the result of ORing the bitmaps of the SDDF sessions. In an embodiment herein, the steps 1372, 1374, 1376 are perfonned using RDF, where an RIfR2 pair is first established using, for example, the Dynamic RDF mechanism disclosed in U.S. patent application no. 091997,810, which is incorporated by reference herein. Following that, the bitmaps may be used to modify device table maps in one or both of the local destination 1204 and the remote destination 1206 to cause the RDF

mechanism to perform a background copy.

Referring to Figure 32, a flow chart 1380 illustrates in more detail steps performed in connection with the terminate ordered writes step 1366 of the flow chart 1360 of Figure 31. Note that the processing of Figure 32 illustrates clean up when the source group 1202 uses a single storage device. In instances where the source group 1202 includes more than one storage device, then the processing illustrated by the Figure 23 may be used instead.

Processing begins at a first test step 1382 where it is determined if all of the ordered writes data received by the remote destination 1206 from the source group 1202 has been completely processed (saved by the remote destination). That is, at the test step 1382, it is determined if there is any data at the remote destination 1206 corresponding to ordered writes cycles that have not been stored on the disk space of the remote destination 1206 or at least in the cache and marked for write to the disk space. If it is determined at the test step 1382 that all ordered wiites data has been processed at the remote destination 1206, then processing is complete.

If it is determined at the test step 1382 that some of the ordered writes data from the source group 1202 has not been processed, then control transfers from the test step 1382 to a test step 1384 which determines if the received, but unprocessed, ordered writes data at the remote destination 1206 corresponds to a complete ordered writes cycle. Note that, as the source group 1202 sends data corresponding to a particular cycle to the remote destination 1206, the cycle at the remote destination 1206 is not complete unless and until a commit message is sent from the source group 1202 to the remote destination 1206. If it is determined at the test step 1384 that the unprocessed data corresponds to a complete ordered write cycle, then control transfers from the test step 1384 to a step 1385 where the data for the cycle is stored by saving the data to the disk of the remote destination 1206 and/or marking the data in the cache of the remote storage device 1206 for write to the disk thereof. Following the step 1385, processing is complete.

If it is determined at the test step 1384 that the unprocessed ordered writes data from the source group 1202 does not correspond to a complete cycle, then control transfers from the test step 1384 to a step 1386 where invalid bits are set on the device table of the remote destination 1206. The invalid bits set at the step 1386 indicate that tracks (or some other data increment) of the remote destination 1206 contain invalid data and thus need to be copied from another device to be brought up-to-date. In this instance, the other device may be the local destination 1204, depending upon which of the local destination 1204 and the remote destination 1206 contains the most up-to-date information. The particular tracks (or other data elements) that are set to invalid in the device table of the remote destination 1206 correspond to the tracks indicated by the unprocessed ordered writes data received from the source group 1202. Following step 1386 is a step 1387 where the unprocessed ordered writes data for the incomplete cycle is discarded. Following step 1387, processing is complete. 7'

Referring to Figure 33, a flow chart 1390 illustrates in more detail steps performed in coImection with the step 1376 where data is copied from the local destination 1204 to the remote destination 1206 or the step 1372 where data is copied from the remote destination 1206 to the local destination 1204. Processing begins at a S first step 1392 where the OR of SDDF 1, SDDF 2, and SDDF_3 (from the step 1374) is used to set a device table at whichever one of the local destination 1204 and the remote destination 1206 will be the RI device after recovery. If data is to be copied from the Ri device to the R2 device, then the device table locations corresponding to remote tracks are set at the step 1392. Otherwise, if data is to be copied from the R2 device to the Ri device, then the device table locations corresponding to local tracks are set at the step 1392. In many instances, the tracks corresponding to the modification of the table at the step 1392 will be the same or a superset of the modification to the table from the step 1386, discussed above.

Following step 1392 is a step 1396 where the background copy process is begun. The background copy process begun at the step 1396 causes data to copied in a background process. Following step 1396, processing is complete.

Referring to Figure 34, a flow chart 1450 illustrates steps performed in connection with reinitializing the recovery parameters once normal operation is restored between the source group 1202, the local destination 1204, and the remote destination 1206. Processing begins at a first step 1452 where both of the SDDF sessions, SDDF_1 and SDDF_2, are deactivated. Following the step 1452 is a step 1454 where SDDF_1 is cleared. Following step 1454 is a step 1456 where SDDF_2 is cleared. Following the step 1456 is a step 1458 with a pointer that points to one of the SDDF sessions is made to point SDDF 1. Following step 1458 is a step 1462 where SDDFj is activated. Following step 1462, processing is complete.

Referring to Figure 35, a flow chart 1470 illustrates steps performed in connection with resetting recovery parameters used by the remote destination 1206.

Processing begins at a first step 1472 where SDDF_3 is deactivated. Following the step 1472 is a step 1474 where SDDF_3 is cleared. Following the step 1474 is a step 1476 where the token used by the remote destination 1206 is cleared (set to zero).

Following the step 1476, processing is complete. Note that, in some embodiments, it is possible to also terminate SDDF_3 at or after the step 1472 so that SDDF_3 may be recreated at the step 1322 of the flow chart 1320 of Figure 29, discussed above.

Referring to Figure 36, a diagram shows a possible configuration of the source group 1202. In the diagram of Figure 36, the source group 1202 includes a host 1502 coupled to a single local storage device 1504. The local storage device 1202 is coupled to both the local destination 1204 and the remote destination 1206. In the configuration shown in Figure 36, all of the processing described herein may be perfonned by the local storage device 1504 or, alternatively, some of the processing may be performed by the host 1502, as appropriate.

Referring to Figure 37, a diagram shows another configuration of the source group 1202 where a host 1512 is coupled to a plurality of local storage devices 1514- 1516. Each of the local storage devices 1514-1516 is coupled to both the local destination 1204 and the remote destination 1206. In the configuration shown in Figure 37, the host 1512 may handle some of the processing described herein such as, for example, cycle switching for all of the local storage devices 1514-15 16 in connection with ordered writes.

In some instances, it may be desirable to prevent cycle switching if there is difficulty with data transfers between the source group 1202 and the local destination 1204. Of course, in instances of complete failure of the local destination 1204 or complete failure of links between the source group 1202 and the local destination 1204, the system may stop working completely and recovery will proceed as described above.

However, in other cases where there may be intermittent failure (e.g., transient connectivity loss for the links between the source group 1202 and the local destination 1204), it may be desirable to simply not cycle switch on the source group 1202 in connection with communication between the source group 1202 and remote destination 1206, unless and until corresponding data is first successfully transferred from the source group 1202 to the local destination 1204. This enhancement is described below.

Referring Figure 38, a flow chart 200' illustrates a modified portion of the flow of Figure 6, which shows steps performed in connection with transferring data from an inactive one of the lists 72, 74 (shown in Figure 3) to another storage device.

The flow chart 200' shows the step 204 and the step 212 which are also in the flow chart 200 and described above in connection with the text that explains Figure 6.

However, as set forth in more detail below, the flow chart 200' contains additional new steps that are not provided in the flow chart 200 of Figure 6.

Following the step 204 is a test step 1602 which determines if the data being transferred to another storage device is special data As used in connection with the flow chart 200', special data at the step 1602 refers to data being transmitted from the source group 1202 to both the local destination 1204 and to the remote destination 1206. In an embodiment herein, special data may be identified using built in mechanisms to determine if the data is queued for transfer to both the local destination 1204 and the remote destination 1206. Of course, other mechanisms for identifying and testing for special data may be used.

If it is determined at the test step 1602 that the data is not special data, then control transfers from the step 1602 to the step 212 to send the data as described above in connection with the flow chart 200 of Figure 6. Following the step 212, processing continues as shown in the flow chart 200 of Figure 6 and described elsewhere herein.

If it is determined that the test step 1602 that the data being sent is special data (is being transferred from the source group 1202 to both the local storage device 1204 and the remote storage device 1206), then control transfers from the test step 1602 to a test step 1604 where it is determined if the particular transfer being examined is a transfer of the special data from the source group 1202 to the remote destination 1206. As discussed elsewhere herein, it is desirable to avoid sending data from the source group 1202 to the remote destination 1206 unless and until the data has first been successfully transferred from the source group 1202 to the local destination 1204. Thus, the test at the step 1604 determines if the specific data transfer being examined is a transfer from the source group 1202 to the remote destination 1206. If not, then control transfers from the test step 1604 to the step 212 to continue processing, as described elsewhere herein.

Otherwise, if it is determined at the test step 1604 that the data being examined corresponds to a transfer from the source group 1202 to the remote destination 1206, then control transfers from the test step 1604 to a test step 1606, which determines if the corresponding transfer of the data from the source group 1202 to the local destination 1204 had previously completed successfully. That is, for a given slot or portion of data being transferred to both the remote destination 1206 and the local destination 1204, the test at the step 1606 determines if the transfer from the source group 1202 to the local destination 1204 was performed successfully. If not, control transfers from the test step 1606 to a step 1607, where error processing/testing are performed.

In some cases, the inability to transfer data from the sourec group 1202 to the local destination 1204 causes the system to begin accumulating data at the remote destination 1206 by, for example, causing a failure message to be sent to the remote destination (see the flow chart 1330 of Figure 30 and corresponding description) and by exiting from the processing illustrated by the flow chart 200' of Figure 38 so that data is sent from the source group 1202 to the remote destination 1206 irrespective of whether the data was ever successfully sent from the source group 1202 to the local destination 1204. Other processing may occur such as, for example, setting invalid bits in a device table for data that is not transferred from the source group 1202 to the local destination 1204. Note that if the connection between the source group1202 and the local destination 1204 is reestablished, it is possible to synchronize the remote destinmation 1204 and then resume steady state operation as described herein (e.g., the steps of the flow chart 200').

The criteria for determining whether or not to perform the above-described error processing may be set according to a number of functional factors discernable by one of ordinary skill iii the art. In an embodiment herein, the criteria is set according to the likelihood that there is a failure of the link between the source group 1202 and the local destination 1204 andlor a failure of the local destination 1204 For example, the error processing at the step 1607 may determine that a failure has occurred if a certain amount of time has passed without data being successfully transferred from the source group 1202 to the local destination 1204. If the error processing at the step 1607 determines that a failure has not occurred (and thus processing should continue), then control transfrrs from the step 1607 to a test step 1608, which determines if there is more inactive data tO be sent from the source group 1202 to the remote destination 1206. If so, then control transfers from the test step 1608 to a step 1612 where a pointer that iterates through the data (e.g., iterates through elements of the inactive one of the lists 74, 76) is adjusted to point to the next inactive block of data to be sent.

Following the step 1612, control transfers back to the step 204, to continue processing as discussed elsewhere herein.

If it is determined at the test step 1608 that there is not more inactive data to be sent, then control transfers from the test step 1608 to a step 1614 where the process waits. Since it has already been determined that the data being sent corresponds to a transfer from the source group 1202 to the remote destination 1206 (at the test step 1604), and it has been established that the corresponding transfer from the source group 1202 to the local destination 1204 has not completed yet (according to the test at the step 1606), then if it is determined at the test step 1608 that there is no more data to be sent, then it is appropriate to wait at the step 1614 so that either more inactive data will be made available to send or until another process successfully transfers corresponding data from the source group 120210 the local destination 1204, thus altering the result at the test step 1606 for the next iteration. Following the step 1614, control transfers back to the step 204 to continue processing as described elsewhere herein.

If it is determined at the test step 1606 that the corresponding transfer to the local destination 1204 had previously completed successfully, then control transfers from the test step 1606 to another test step 1616 to determine if the data being transferred has more than one slot associated therewith in connection with the transfer (e.g., an active slot and an inactive slot). As discussed elsewhere herein, under certain conditions, it is possible for there to be more than one slot associated with what would otherwise be a single slot. See, for example, the discussion above in connection with the flow chart 440 of Figure 12 and the steps 446, 472, 474, 476, 478. Thus, if it is determined at the test step 1616 that there is not more than one slot, then control transfers from the test step 1616 to the step 212, to continue processing as described elsewhere herein. Otherwise, if it is determined at the test step 1616 that there is more than one corresponding slot, then control transfers from the test step 1616 to a test step 1618, which determines if the transfer of the other slots to the local storage device 1204 had been successful, like the test for the slot at the step 1606. If it is determined at the test 1618 that all of the other corresponding slots were transferred properly to the local storage device 1204, then control transfers from the test step 1618 to the step 212 to continue processing as described elsewhere herein. Otherwise, control transfers from the test step 1618 to the step 1608, also described elsewhere herein.

In another embodiment of the system described herein, it is possible to not use COVD's for the R2 device like those shown in the diagram 240 of Figure 7. That is, it is possible to implement the R2 receipt of asynchronous data without using COVD's at the R2 device.

Referring to Figure 39, a diagram 1640 shows a cache 1642 that is provided in the remote storage device 26 that receives data. The cache 1642 includes a plurality of slots 1652-1654 in which asynchronous data that is received from the local storage device 24 is placed. Also shown is a first circularly linked list 1674 and a second circularly linked list 1676 which contain pointers to the slots 1652-1654 of the cache 1642. Thus, for example, the circularly linked list 1674 includes a plurality of pointers 1681-1685, each of which points to one of the slots 1652-1654 in the cache 1642.

Similarly, the circularly linked list 1676 includes a plurality of pointers 1691-1695, each of which points to one of the slots 1652-1654 of the cache 1642. A standard logical device 1698 is also mapped to portions of the cache 1642.

In an embodiment herein, one of the lists 1674, 1676 corresponds to an inactive data chunk (e.g., like the chunk 56 shown in Figure 2), while the other one of the lists 1674, 1676 corresponds to an active data chunk (e.g., like the chunk 58 of Figure 2).

Received data is accumulated using an inactive one of the data chunks while the active one of the data chunks is used for stonng data at the standard logical device 1698 as described elsewhere herein in connection with the diagram 240 of Figure 7 and the corresponding text. Thus, as new data arrives, it is placed in the cache 1642 and a new pointer is added to which one of the circularly linked lists 1674, 1676 corresponds to the inactive data chunk when the data is received.

In some instances, it may be useful to be able to determine whether a portion of the standard logical device 1698 (or any other logical device) has a slot associated therewith in the cache 1642 corresponding to received data. Of course, it is always possible to traverse both of the lists 1674, 1676 to determine if there is a corresponding slot in the cache 1642. However, it would be more useful if there were a way of using particular device, cylinder, and head values of a logical device to determine whether there is a corresponding one of the slots 1652-1654 in the cache 1642 waiting to be destaged to the device.

Referring to Figure 40, a diagram 1700 shows a hash table 1702 which contain a plurality of entries 1704-1706. In an embodiment herein, each of the entries 1704- 1706 either contains a null pointer or points to one of the cache slots 1652-1654 that correspond to data that has been received but not yet stored on the standard logical device 1698 (or another standard logical device). The table 1702 is indexed using a hash function that performs a mathematical operation using the particular values of the device, cylinder, and head to generate an index into the table 1702 to find the corresponding entry. Thus, when data is received by the R2 device, the hash function is applied to the device, cylinder, and head to find its index value into the table 1702 and then a pointer is written to the corresponding one of the entries 1704-1706 that points to the particular slot 1652-1654 in the cache 1642. Once the received data is appropriately destaged to the standard logical device 1698 (or another device), the corresponding one of the entries 1704-1706 is set to null. In this way, the hash table 1702 allows quickly determining whether a particular portion of a standard logical device corresponds to received data that has not yet been destaged. For the system described herein, any appropriate hash function may be used to generate the index into

the table 1702.

In some instances, it may possible for a particular device, cylinder, and head values to generate an index into the table 1702 that is the same as an index generated by different values for the device, cylinder, and head. This is called a "collision". In instances where collisions occur, a second entry into the table 1702 corresponding to the same index as provided and the second entry is linked to the first entry so that a particular index would correspond to more than one entry. This is illustrated by an element 1708 that is linked to the element 1705 of the table 1702. thus, a first device, cylinder, and head are hashed to generate and index to the entry 1 705 while different device, cylinder, and head are input to the hash function to generate the same value for the index. In an embodiment herein, the entry 1705 is used to point to the data in the cache 1642 corresponding to the first device, cylinder, and head while the entry 1708 is used to point to data in the cache 1642 corresponding to the second device, cylinder and head. Of course, as data is desthged to an appropriate device, the corresponding one of the entries 1705, 1708 may be eliminated from the table 1700.

Note that any number of entries may correspond to a single index so that, for example, if collisions occur that cause three separate sets of values for device, cylinder, and head to generate the same index, then there would be three (or more) entries linked together at a particular index into the table 1702. Note also that other appropriate techniques may be used to handle collisions, including providing additional tables (e.g., a second table, a third table, a fourth table, etc.).

Referring to Figure 41, a diagram 1720 shows an alternative embodiment of a hash table 1722 which contain a plurality of entries 17244726. The embodiment of Figure 41 is like the embodiment of Figure 40, with a few differences, as described herein. Each of the entries 17241726 either contains a nuJi pointer or points to one of the cache slots 1728, 1732, 1734, shown in the diagram 1720, that correspond to data that has been received but not yet stored on the standard logical device 1698 (or another standard logical device). The table 1722 is indexed using a hash function that performs a mathematical operation using the particular values of the device, cylinder, and head to generate an index into the table 1722 to find the corresponding entry. Thus, when data is received by the R2 device, the hash function is applied to the device, cylinder, and head to find its index value into the table 1722 and then a pointer is written to the corresponding one of the entries 1724-1726 that points to the particular slot 1728, 1732, 1734. Once the received data is appropriately destaged to the standard logical device 1698 (or another device), the corresponding one of the entries 1724-1726 is adjusted appropriately. In this way, the hash table 1722 allows quickly determining whether a particular portion of a standard logical device corresponds to received data that has not yet been destaged. For the system described herein, any appropriate hash function may be used to generate the index into the table 1722.

For the embodiment shown in Figure 41, in instances where collisions occur, the first slot pointed to by a table entry points to the second slot that caused the collision. Thus, for example, if the slot 1732 and a slot 1736 cause a collision at the table entry 1725, the table entry 1725 points to the slot 1732 while the slot 1732 points to the slot 1736. Thus, a collision does not cause any change in the table 1722 when the subsequent slot is added, since adding the subsequent slot simply involves changing the pointer value for a previous slot. Of course, any number of slots may correspond to a single table entry.

In some instances, it may be advantageous to be able to use something other than the local destination 1204 shown in the diagram 1200 of Figure 26. Of course, if the local destination 1204 is not a full mirror of the source group, then recovery would be performed at the remote destination 1206. However, the remote destination 1206 may not contain data that is as up-to-date as data that was synchronously written from the source group 1202 to the local destination 1204. Accordingly, it is desirable to be able to recover at the remote destination 1206 but have data that is as up-to-date as data synchronously written from the source group 1202 to the local destination 1204.

S Referring to Figure 42, a diagram 1780 shows a system containing a source group 1782 like the source group 1202 of the diagram 1200 of Figure 26. The diagram 1780 also shows a remote dcstination 1786 like the remote destination 1206 of the diagram 1200 of Figure 26.

The diagram 1780 also shows a minimal storage local destination 1788, which is used to receive synchronous data writes from the source group 1782. In an embodiment herein, the minimal storage local destination 1788 appears to the source group 1782 to be a storage device like the local destination 1204 of the diagram 1200 of Figure 26. However, in an embodiment herein, the minimal storage local destination 1788 is not a mirror of the source group 1782. Instead, the minimal storage local destination 1788 is used to store data that will be transferred to the remote destination 1786 in the event that a recovery becomes necessary. In an embodiment herein, all recoveries are performed at the remote destination 1786 In other embodiments, it is possible for the source group 1782 to know that it is connected to the minimal storage local destination 1788.

The source group 1782 may operate like the source group 1202 of the diagram 1200 of Figure 26. Thus, the source group 1782 provides synchronous writes to the minimal storage local destination 1788 and provides ordered writes to the remote destination 1786. The minimal storage local destination 1788, instead of being a minor for the source group 1782, stores only the most recent synchronous writes from the source group 1782 so that, in the event a recovery becomes necessary, the minimal storage local destination 1788 may transfer the most recent writes to the remote destination 1 786. As data is transferred from the source group 1782 to the remote destination 1786, corresponding data is removed from the minimal storage local destination 1788 so that the minimal storage local destination 1788 retains, for example, only the most recent two cycles of ordered writes data because any cycles older than the two most recent cycles are already stored on the remote storage device 1786.

Referring to Figure 43, a data structure 1800 illustrates storage of data at the minimal storage local destination 1788. In an embodiment herein, the data structure 1800 is a linked list having a plurality of elements 1802-1805. A list top pointer points to the first element 1802. The first element points to the second element 1803, the second element points to the third element 1804, et cetera. The linked list 1800 also has a last element 1805 that points to a null pointer as a next element to indicate the end of the list. Of course, other appropriate data structures may be used.

In one embodiment herein, the list 1800 is sorted according to the track and head location on a standard logical device so that, for example, the element 1802 would be the first element in a sequence of elements on the standard logical device, the second element 1803 would follow the first element 1802, et cetera. In another embodiment, the list 1800 would be sorted according to the order that the data writes are provided from the source group 1782 to the minimal storage local destination 1 788. Both embodiments are described in more detail below.

Referring to Figure 44, a diagram shows in more detail a data element 1820 of the linked list 1800. The data element 1820 includes a TRACK field 1822 that indicates a track number used to store the data on the corresponding standard logical device. Other data storage locations may be used in lieu of track number, such as sector number, offset from beginning of the standard logical device, or any other appropriate indicator.

The data element 1 820 also includes a cycle number 1824 (CYCNO) which indicates the cycles used for transferring data from the source group 1782 to the remote destination 1786, as described elsewhere herein. When data is transferred from the source group 1782 to the minimal storage local destination 1788, the minimal storage local destination 1788 keeps track of cycle changes corresponding to transfers between the source group 1782 and the remote destination 1786 and assigns an appropriate cycle number to the data element 1820, as described in more detail elsewhere herein.

The data element 1820 also includes a data field 1826 (DATA) that corresponds to the data that has been transferred from the source group 1782 to the minimal storage local destination 1788. The data element 1820 also includes a next field 1828 (NEXT) that is used to point to the next data element in the linked list 1800.

Referring to Figure 45, a flow chart 1850 illustrates steps performed by the minimal storage local destination 1788 in connection with receiving data provided by the source group 1782 to the minimal storage local destination 1788. As discussed elsewhere herein, the minimal storage local destination 1788 may appear to the source group 1782 to be a mirror of the source group 1782. However, as described herein, the minimal storage local destination 1788 is not necessarily a mirror of the source group 1782 but, instead, contains the most recent data from the source group 1782. In the event a recovery is necessary, the minimal storage local destination 1788 transfers the data stored thereon to the remote destination 1786 to facilitate recovery.

Processing for the flow chart 1850 begins at a step 1852 where a local cycle number, CYCNO, is set to zero. The local cycle number set at the step 1852 is used to keep track of the cycle changes at the source group 1782. Note that it is not necessary that the local cycle number correspond exactly to the particular cycle number used at the source group 1782. Rather, it is sufficient that the cycle number set at the step 1852 keep track of the cycle switches by the source group 1782.

Following the step 1852 is a step 1854 where data is received from the source group 1782. Following the step 1854 is a test step 1856 which determines if the received data corresponds to a cycle switch provided by the source group 1782. The test step 1856 is like the test step 1262 of Figure 28A, discussed above.

If it is determined at the test step 1856 that the received data does not correspond to a cycle switch, then control transfers from the test step 1856 to a step 1858 where the received data is added to the storage of the minimal storage local destination 1788. Adding the data to the storage of the minimal storage local destination 1788 at the step 1858 is discussed in more detail elsewhere herein.

Following step 1858 is a step 1862 where the received data is acknowledged to the source group 1782 by the minimal storage local destination 1788. Acknowledging the data at the step 1862 may be used to make the minimal storage local destination 1788 appear to the source group 1782 as a full synchronous mirror (i.e., the acknowledge provided at the step 1862 is the same acknowledge provided by a full synchronous mirror). Following the step 1862, control transfers back to the step 1854 to receive more data.

If it is determined at the test step 1856 that the data received at the step 1854 corresponds to a cycle switch, then control transfers from the step 1856 to a step 1864 where the internal cycle number, CYCNO, that is used with the minimal storage local destination 1788, is incremented. In an embodiment herein, there is a maximum cycle number, MAXCYCNO, so that the cycle number is incremented at he step 1864 by adding one to CYCNO and then taking the result thereof modulo MAXCYCNO.

Following the step 1864 is a step 1866 where data corresponding to previous cycle numbers is discarded. As discussed elsewhere herein, the minimal storage local destination 1788 stores the data corresponding to the two most recent cycle numbers.

Any data older than two cycle numbers should have already been successfully transferred from the source group 172 to the remote destination 1786. Accordingly, since recovery will be provided at the remote destination 1786, then the minimal storage local destination 1788 need only store data corresponding to the current cycle number and data corresponding to the previous cycle number. In other embodiments, it may be possible to retain different data (e.g., the three or four most recent cycles).

Discarding data at the step 1866 is discussed in more detail elsewhere herein.

Following the step 1866, control transfers back to the step 1854, discussed above, to receive additional data.

Referring to Figure 46, a flowchart 1900 illustrates in more detail steps performed by thc minimal storage local destination 1788 in connection with the step 1858 where received data is added to the data stored at the minimal storage local destination 1788. Processing begins at a first step 1902 where a temporary variable, P1, is set equal to list top, the pointer that points to the linked list 1800 of the data stored at the minimal storage local destination 1788. -Following the step 1902 is a test step 1904 where it is determined if the temporary variable, P1, equals null. If so, then there is no data stored in the linked list 1800 and the data being stored by the steps of the flowchart 1900 corresponds to the first data being stored at the minimal storage local destination 1788. If it is detennined at the test step 1904 that P1 equals null, then control transfers from the test step 1904 to a step 1906 where memory is obtained from a heap (or something appropriate similar) using a malloc command (or something appropriately similar) for a temporary variable, 1, used to temporarily store data. Following the step 1906 is a step 1908 where a track number field of the data record T (T.TRACK) is set equal to the track of the standard logical device on which the data is stored at the source group 1782. Following the step 1908 is a step 1912 where T.CYCNO is set equal to the current local cycle number at the minimal storage local destination 1778.

Following the step 1912 is a step 1914 where the received data is copied to the data field of T (i.e., T.DATA). Following the step 1914 is a step 1916 where the next field (T.NEXT) is set equal to null. Following the step 1916 is a step 1918 where the list top is set equal to T. Following the step 1918, processing is complete.

If it is determined at the test step 1904 that the variable P1 does not equal null, then control transfers from the test step 1904 to a test step 1922 where it is determined if the track number corresponding to the data that has been received equals the track number of the data element pointed to by P1. If so, then control transfers from the test step 1922 to a step 1924 where the received data is written to the data field at P1.DATA (perhaps overwriting existing data). Following the step 1924 is a step 1926 where the cycle number field of the element pointed to by P1 (P1.CYCNO) is overwritten with the current local cycle number (CYCNO) stored at the minimal storage]ocal destination 1778. Following the step 1926, processing is complete.

If it is determined at the test step 1922 that they track number of the element pointed to by P1 does not equal the track number corresponding to the received data, then control transfers from the test step 1922 to a step 1928 where another temporary variable, P2, is set equal to P1.NEXT. Following the step 1928 is a test step 1932 where it is determined if P2 equals null (i.e., P1 points to an element at the end of the linked list 1 800). If not, then control transfers from the test step 1932 to a test step 1934 where it is determined if the track number corresponding to the element pointed to by P2 is less than the track number corresponding to the data that has been received.

The test at the step 1934 determines if the received data is to be a new element that is interposed between P1 and P2. If it is determined at the test step 1934 that the track number of the element pointed to by P2 is not less than a track number corresponding to the received data, then control transfers from the test step 1934 to a step 1936 where the temporary variable P1 is set equal to P2 in order to prepare for the next iteration.

Following the step 1936, control transfers back to the test at 1922, discussed above.

If it is determined at the test step 1932 that P2 points to null, or if it is determined at the test step 1934 that the track number of the data element pointed to by P2 is less than the track number corresponding to the received data, then control transfers to the step 1938 where a new element (for the list 1800) is allocated using a temporary variable, T. Following the step 1938 is a step 1942 where the track number of T (T.TRACK) is set equal to the track number corresponding to the received data.

Following the step 1942 is a step 1944 where the cycle number field of T (T.CYCNO) is set equal to the local cycle number at the minimal storage local destination 1788.

Following the step 1944 is a step 1946 where the received data is written to the data field of the temporary storage area (T.DATA). Following the step 1946 is a step 1948 where the next field of I (T.NEXT) is set equal to P2. Following the step 1948 is a step 1952 where the next field of the element pointed to by P1 (P1.NEXT) is set to point to T T. Following the step 1952, processing is complete. -86

The flowchart 1900 of Figure 46 illustrates an embodiment where the linked list 1800 at the minimal storage local destination 1788 is sorted according to the track number corresponding to the received data. It is also possible to sort the received data according to the order of receipt of the data at the minimal storage local destination 1788.

Referring to Figure 47, a flowchart 2000 illustrates steps performed by the minimal storage local destination 1788 at the step 1858 where data is added. In the alternative embodiment illustrated by the flowchart 2000, the data in the linked list 1800 is stored according to the order of receipt of the data at the minimal storage local destination 1788.

Processing begins at first step 2002 where a new data element, T, is allocated.

Following the step 2002 is a step 2004 where the track number field of the allocated element (T.TRACK) is set equal to the track number corresponding to the received data. Following the step 2004 is a step 2006 where the cycle number field of the allocated element (T.CYCNO) is set equal to the local cycle number provided at the minimal storage local destination 1788 (CYCNO).

Following the step 2o06 is a step 2008 where the received data is written to the data field of the allocated element (T.DATA). Following the step 2008 is a step 2012 where the next field of the allocated element (T.NEXT) is set equal to null. In an embodiment herein, the most recently received data is added at the end of the linked list 1800. Following the step 2012 is a test step 2014 which determines if the list top (the pointer to the head of the linked list 1800) equals null. Note that whenthe first data is written to the linked list 1800, or if all data has been removed from the list 1800, the list top may equal null. If it is determined at the test step 2014 that the list top equals null, then control transfers from the test step 2014 to a step 2016 where the list top is set to point to the newly allocated data element, T. Following the step 2016 is a step 2018 where a list end element, LE, which keeps track of the end of the list 1800, is also set to point to the newly allocated data element, T. Following the step 2018, processing is complete.

If it is determined at the test step 2014 that the list top does not equal null, then control transfers from the test step 2014 to a step 2022 where the next field of the data element pointed to by the list end pointer (LE.NEXT) is set to point to the newly allocated data element, T. Following the step 2022 is a step 2024 where the list end pointer, LE, is set equal to the newly allocated data element, T. Following the step 2024, processing is complete.

Referring to Figure 48, a flow chart 2050 illustrate steps performed by the minimal storage local destination 1788 in connection with the discard step 1866 of the flow chart 1850 of Figure 45. In an embodiment herein, the same processing may be used for discarding irrespective of whether the data is stored on the linked list 1800 according to the track number (embodiment of Figure 46) or according to the order of receipt (embodiment of Figure 47). Note that a technique used for discarding the data may be independent of a technique used to add data.

Processing for the flow chart 2050 begins at a first step 2052 where a temporary variable, P1, is set equal to the list top variable that points to the first element of the Jinked list 1800. Following the step 2052 is a test step 2054 where it is determined if P1 equals null. If so, then processing is complete. Otherwise, control transfers from the test step 2054 to a step 2056 where another temporary variable, P2, is set equal to the next field of the data element pointed to by P1 (i.e., is set equal to P1.NEXT).

Following the step 2056 is a test step 2058 where it is determined if P2 points to null.

If so, then processing is complete. Otherwise, control transfers from the test step 2058 to a test step 2062 which determines if the cycle number of field of the data element pointed to by P2 (P2.CYCNO) equals the cycle number for data being discarded (e.g., (CYCNO-2) modulo MAXCYCNO). If so, then control transfers from the test step 2062 to a step 2064 where the next field of the element pointed to by P1 (P1.NEXT) is set equal to the next field of the element pointed to by P2 (P2.NEXT), thus removing the element pointed to by P2 from the linked list 1800. Following the step 2064 is a step 2066 where the element pointed to by P2 is freed (i.e., returned to a heap).

Following the step 2066 is a step 2068 where P1 is set equal to the next field pointed to by P1 (i.e., P1= P1.NEXT). Following the step 2068, control transfers back to the step 2056 for the next iteration. Note that if it is determined at the test step 2062 that the cycle number field of the element pointed to by P1 does not equal a cycle number of data being discarded, then control transfers from the test step 2062 to the step 2068, discussed above.

Note that for the embodiments discussed in connection with Figures 42-48, it is not necessary to maintain the SDDF maps since recovery is always performed at the remote destination 1786 and all of the stored data is transferred from the minimal storage local destination 1788 to the remote destination 1786 recovery. Thus, it is not necessary to keep track of specific data that needs to be transferred. In addition, it is not necessary to have a token or any other mechanism for keeping track of which device will be used for recovery since the remote destination 1786 is always used for recovery in connection with the embodiments of Figures 42-48. Furthermore, it is not necessary to perform the processing of Figures 29 and 30 at the remote destination since there are no tokens or SDDF maps.

Referring to Figure 49, a flow chart 2100 illustrates steps performed in connection with recovering at the remote destination 1786 when, for example, the source group 1782 ceases to be operational. Processing begins at a first step 2102 where the links between the source group 1782, the remote destination 1786 and the minimal storage local destination 1788 are dropped. Dropping the links at the step 2102 is like dropping the links at the step 1362 of the flow chart 1360 of Figure 31.

Following the step 2102 is a step 2104 where ordered writes are terminated.

Terminating ordered writes at the step 2104 is like terminating ordered writes at the step 1366 of the flow chart 1360 of Figure 31.

Following the step 2104 is a step 2106 where all data is transferred from the minimal storage local destination 1788 to the remote destination 1786. The data may be transferred according to the order of data elements on the linked list 1800. Thus, for example, if the linked list 1800 is stored according to the time sequence of writes to the minimal storage local destination 1788 (embodiment shown in Figure 47), then the data will be transferred from the minimal storage local destination 1788 to the remote destination 1786 according to the time sequence of writes. Alternatively, if data is stored in the linked list 1 800 in order of storage locations on the corresponding standard logical device (embodiment if Figure 46), then data may be transferred in that order from the minimal storage local destination 1788 to the remote destination 1786.

Following the step 2106, processing is complete and operation may resume using the remote storage device 1786, which has the most up-to-date data.

Referring to Figure 50, a diagram 2120 illustrates sharing of resources. The diagram 2120 includes a source group 2122 and a remote destination 2126 which are like the source group 1782 and the remote destination 1786 of Figure 42. The diagram 2120 also shows a minimal storage local destination 2128 which is like the minimal storage local destination 1788 of Figure 42, except that the minimal storage local destination 2128 may be shared by unrelated storage systems. Note that the minimal storage local destination 2128 may be any type of storage device including a Symmetrics device provided by EMC Corporation, a personal computer, or any other device capable of storing data and providing the functionality described herein.

The diagram also shows a second source group 2126' and a second remote destination 2122'. However, the source group 2122' transfers data to the minimal storage local destination 2128 that is also used by the source group 2122. Should recovery become necessary for either or both of the source groups 2122, 2122', recovery data will be provided by the minimal storage local destination 2128 to one or both of the remote destinations 2126, 2126'. In an embodiment herein, the minimal storage local destination 2128 stores data from the source group 2122 in a separate location from data stored for the source group 2122'.

The diagram 2120 also shows a third source group 2122" and a third remote destination 2126". The third source group 2122" also transfers data to the minimal storage local destination 2128. In addition, should recovery become necessary, the minimal storage local destination 2128 may transfer recovery data to the remote destination 2126".

The number of source groups and remote destination coupled to a minimal storage local destination may be bounded by the storage and processing capability of the minimal storage local destination. Note also that a single storage device may be used as the remote destination for multiple source groups.

While the invention has been disclosed in connection with various embodiments, modifications thereon will be readily apparent to those skilled in the art.

Accordingly, the spirit and scope of the invention is set forth in the following claims.

Claims

CLAJIvIS 1. A method for storing recovery data, comprising: providing

chunks of data to a remote destination, wherein each chunk of data represents data written before a first time and after a second time and wherein the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks; providing synchronous data corresponding to the chunks of data to a minimal storage local destination, wherein the minimal storage local destination contains data that is not stored on the remote destination and wherein the remote destination contains data that is not stored on the minimal storage local destination; and providing an indicator when a new chunk of data is being provided.

2. A method, according to claim 1, wherein the data that is contained on the remote destination but not on the minimal storage local destination is older than the data that is contained on the minimal storage local destination but not on the remote destination.

3. A method, according to claim 1, wherein the synchronous data received by the minimal storage local destination is stored in a linked]ist of elements.

4. A method, according to claim 3, wherein the elements are sorted according to a source storage device location of the data.

F

5. A method, according to claim 3, wherein the elements are sorted according to an order of receipt of the data at the minimal storage local destination.

6. A method, according to claim 3, wherein each of the elements includes an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements.

7. A method, according to claim 1, further comprising: providing a local cycle number at the minimal storage remote destination; and in response to receiving the indicator that a new chunk of data is being provided, incrementing the local indicator.

8. A method, according to claim 7, wherein the synchronous data received by the minimal storage local destination is stored in a linked list of elements.

9. A method, according to claim 8, wherein each of the elements includes an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements and wherein the identifier corresponds to the local cycle number.

10. A computer program, provided in a computer readable medium, that stores recovery data, comprising: executable code that provides chunks of data to a remote destination, wherein each chunk of data represents data written before a first time and after a second time and wherein the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks; executable code that provides synchronous data corresponding to the chunks of data to a minimal storage local destination, wherein the minimal storage local destination contains data that is not stored on the remote destination and wherein the remote destination contains data that is not stored on the minimal storage local destination; and executable code that provides an indicator when a new chunk of data is being provided.

11. A computer program Computer software, according to claim 10, wherein the data that is contained on the remote destination but not on the minimal storage local destination is older than the data that is contained on the minimal storage local destination but not on the remote destination.

12. A computer program according to claim 10, wherein the synchronous data received by the minimal storage local destination is stored in a linked list of elements.

1 3. A computer program according to claim 12, wherein the elements are sorted according to a source storage device location of the data.

14. A computer program Computer software, according to claim 12, wherein the elements are sorted according to an order of receipt of the data at the minimal storage local destination.

15. A computer program Computer software, according to claim 12, wherein each of the elements includes an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements.

16. A computer program Computer software, according to claim 10, further comprising: executable code that provides a local cycle number at the minimal storage remote destination; and executable code that increments the local indicator in response to receiving the indicator that a new chunk of data is being provided.

17. A computer program Computer software, according to claim 16, wherein the synchronous data received by the minimal storage local destination is stored in a linked list of elements.

18. A computer program Computer software, according to claim 17, wherein each of the elements includes an identifier for a particular one of the chunks of data corresponding to the synchronous data of each of the elements and wherein the identifier corresponds to the local cycle number.

19. A system for storing recovery data, comprising: a source group; a remote destination coupled to the source group to receive therefrom chunks of data, wherein each chunk of data represents data written before a first time and after a second time and wherein the second time for one of the particular chunks corresponds to a first time for a subsequent one of the particular chunks; and a minimal storage local destination coupled to the source group to receive synchronous data therefrom, wherein, the minimal storage local destination contains data that is not stored on the remote destination and the remote destination contains data that is not stored on the minimal storage local destination and wherein the source group provides an indicator when a new chunk of data is being provided 20. A system, according to claim 19, wherein the data that is contained on the remote destination but not on the minimal storage local destination is older than the data that is contained on the minimal storage local destination but not on the remote destination.

21. A method substantially as described herein.

22. A method substantially as described herein with reference to any one of Figures 1 to 50.

23. A system substantially as described herein.

24. A system substantially as described herein with reference to any one of Figures 1 to 50.

25. A computer program substantially as described herein.

26. A computer program substantially as described herein with reference to any one of Figures 1 to 50.

27. A computer program product substantially as described herein.

28. A computer program product substantially as descnbed herein with reference to any one of Figures ito 50.