James Bottomley, on 07/28/2010 06:37 PM wrote:
On Wed, 2010-07-28 at 16:23 +0200, Tejun Heo wrote:
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?
No. That requires support for QErr ... which is in the same mode page.
The real reason we have difficulty is that BUSY/QUEUE_FULL can cause
reordering in the issue queue, which is a driver problem and not in the
SCSI standards.
BTW, I for long time wandering why low level drivers should process
BUSY/QUEUE_FULL and perform adjusting the queue depth. Isn't it common
for the drivers, so should be performed on the higher (SCSI) level? This
level would provide facility to prevent reordering, if needed, and the
driver would communicate with it in a transparent level.
I mean the following. A driver always deals with a single command at
time. It either sends the command to the device, or sends command's
status/sense from the device to the SCSI level. Then SCSI level decides
if to send another command to the driver or perform necessary recovery,
eg, adjusting queue depth or using ACA restarting the QUEUE_FULL'ed command.
In this architecture there would not be needed to update all the drivers
to provide ordering guarantees and ACA based recovery as it seems needed
now.
Or, am I missing something?
Thanks,
Vlad
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