http://bugzilla.kernel.org/show_bug.cgi?id=15565 --- Comment #2 from Mike Hayward <mh-linux-kernel@xxxxxxxx> 2010-03-18 08:23:55 --- Hey Doug, > bugzilla-daemon@xxxxxxxxxxxxxxxxxxx wrote: > > http://bugzilla.kernel.org/show_bug.cgi?id=15565 > > > > Summary: SCSI Generic queueing completes commands in reverse > > order > > I have been told that is a feature :-) > > The SCSI mid level processes commands from > pass-throughs (e.g. sg and bsg) in LIFO order. > For certain types of error processing it makes > sense. For READs and WRITEs it makes no sense. Thanks for the confirmation; it in fact it makes anti sense to a spinning disk drive! Without drive command queueing it's causing the maximum rotational delay for reads or writes with caching disabled, maximizing latency and minimizing throughput to a given track. Ouch. I assume libaio does not exhibit this behavior, or is everything running through the scsi mid level and just depending on adjacent io aggregation to help reduce rotational latency? I'm guessing I'll have to use libaio to get good disk performance in the near future and save sg for controlling caching, etc. Bummer since SCSI gives much better control, like priority ios to a journal, which NCQ can actually grant and SCSI-ATA translation should support or SCSI drive can honor closer to the media. Or FUA & DPO, or command ordering to pipeline a barrier. - Mike -- Configure bugmail: http://bugzilla.kernel.org/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are the assignee for the bug. -- 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
