Shooting at SCSI targets

By Jonathan Corbet
December 22, 2010

The SCSI protocol normally specifies a two-sided conversation between an "initiator," which initiates requests, and a "target," which acts upon those requests. Normally, the initiator is the host computer, while the target is a storage device; it's thus not surprising that the bulk of the SCSI-related code in the kernel implements the initiator role. There are times, though, when it is useful to have a Linux system act as a SCSI target, usually when that system is the interface to some sort of complicated storage array. The target mode is currently implemented by the STGT code, but that subsystem has been seen as being ripe for replacement for some time.

There are two main contenders to replace STGT: LIO and SCST. In the end, there's really only room in the kernel for one SCSI target implementation, so there naturally has been a fair amount of tension between these two projects. Whenever the discussion turned to choosing one, it tended toward the ugly side. SCSI maintainer James Bottomley has done his best to stay out of the flames, but, in the end, he must make a decision and merge one of them.

A few months back, it began to become clear that LIO was going to be the winner. More recently, James gave the green light to begin merging this code for the 2.6.38 kernel. Suffice to say that SCST maintainer Vladislav Bolkhovitin did not take the decision well and did his best to restart the battle in a wider context. James has stuck with his decision, though, saying that there is not much to choose between technically, and that it came down to community:

Or said a different way: as long as you choose the most community oriented of competing offerings, the community will fill any perceived gaps. Conversely, you can destroy a project simply by alienating the community. That's why community is more important than feature set.

The previous discussions appear to have worn down most other participants, so few people chose to join in this time around. There doesn't seem to be anything to suggest that the decision will change at this point; unless something surprising happens, LIO will be the in-kernel SCSI target subsystem as of 2.6.38.

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Shooting at SCSI targets

Posted Jan 6, 2011 4:43 UTC (Thu) by kev009 (guest, #43906) [Link]

This was stupid. SCST is far more mature as a general purpose target. LIO is an iSCSI target that made a token effort to add some hardware drivers to quell concern about it being merged over SCST. They even admit on their wiki the Qlogic driver was based on the SCST implementation. That's fine, it's open source, but what point does reimplementing all the drivers SCST has developed over a decade serve? For all intents and purposes, SCST is a better choice. The development of LIO will fizzle out for anything but iSCSI after it is merged because it doesn't make business sense to RisingTides. Meanwhile, SCST has been used in products and production for years. WTF?