Hello, On 07/28/2010 03:55 PM, Vladislav Bolkhovitin wrote: >> The only benefit of doing it in the block layer, and probably the >> reason why it was done this way at all, is making use of advanced >> ordering features of some devices - ordered tag and linked commands. >> The latter is deprecated and the former is fundamentally broken in >> error handling anyway. > > Why? SCSI provides ACA and UA_INTLCK which provide all needed > facilities for errors handling in deep ordered queues. I don't remember all the details now but IIRC what was necessary was earlier write failure failing all commands scheduled as ordered. Does ACA / UA_INTLCK or whatever allow that? >> Furthermore, although they do relax ordering >> requirements from the device queue side, the level of flexibility is >> significantly lower compared to what filesystems can do themselves. > > Can you elaborate more what is not sufficiently flexible in SCSI > ordered commands, please? File systems are not communicating enough ordering info to block layer already so we already lose a lot of ordering information there and SCSI ordered queueing is also pretty restricted in what kind of ordering it can represent. The end result is that we don't gain much by using ordered queueing. It may cut down command latencies among commands used for barrier sequence but if you compare it to the level of parallelism filesystem code can exploit by ordering requests themselves... Another thing is coverage. We have ordered queueing for quite some time now but there are only a couple of drivers which actually support them. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
