Steve Byan wrote:
On Mar 2, 2006, at 11:21 AM, Vladislav Bolkhovitin wrote:
Could anyone advice how a SCSI target device can IO-throttle its
initiators, i.e. prevent them from queuing too many commands, please?
I suppose, the best way for doing this is to inform the initiators
about the maximum queue depth X of the target device, so any of the
initiators will not send more than X commands. But I have not found
anything similar to that on INQUIRY or MODE SENSE pages. Have I
missed something? Just returning QUEUE FULL status doesn't look to be
correct, because it can lead to out of order commands execution.
Returning QUEUE FULL status is correct, unless the initiator does not
have any pending commands on the LUN, in which case you should return
BUSY. Yes, this can lead to out-of-order execution. That's why tapes
have traditionally not used SCSI command queuing.
Look into the unit attention interlock feature added to SCSI as a
result of uncovering this issue during the development of the iSCSI
standard.
Apparently, hardware SCSI targets don't suffer from queuing overflow
and don't return all the time QUEUE FULL status, so the must be a way
to do the throttling more elegantly.
No, they just have big queues.
Thanks for the reply!
Things are getting clearer for me now, but still there are few things
that are not very clear for me. Hope, they won't require too long
answers. I'm asking, because we in SCST project (SCSI target mid-level
for Linux + some target drivers, http://scst.sourceforge.net) must
emulate correct SCSI target device behavior under any IO load, including
extreme high one.
- Can you estimate, please, how big target commands queue should be in
order to initiators will never receive QUEUE FULL status? Considering
case that initiators are Linux-based and each has a separate and
independent queue.
- The queue could be so big that the last command in it could not been
processed before the initiator's timeout, then, after the timeout was
hit, the initiator would start issuing ABORTs for the timeouted command.
Is it OK behavior? Or rather misconfiguration (of who, initiator or
target?)? Does the initiator in such situation supposed to reissue the
command after the preceding ones finished, or behave somehow else?
Apparently, ABORTs must hit the performance at the similar degree as too
many QUEUE FULLs, if not more.
Seems, we should setup on the target queue with virtually unlimited size
and, if an initiator is dumb enough to queue so much commands that there
will be timeouts, then it will be its problem and duty to rule the
situation without performance loss. Does it looks OK?
Thanks,
Vlad
Regards,
-Steve
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