Steve Byan wrote:
On Mar 7, 2006, at 12:53 PM, Vladislav Bolkhovitin wrote:
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.
Do you have a per-target pool of resources for handing commands, or are
the pools per-logical unit?
Most limited resource is memory allocated for commands buffers. It is
per-target. Other resourses, like internal commands structures, are so
small, so they could be considered virtually unlimited. They are also
global, but accounting is done by per-(session(nexus), LU).
I'm not sure you could size the queue so that TASK_SET_FULL is never
returned. Just accept the fact the the target must return TASK_SET_FULL
or BUSY sometimes.
We have relatively cheap method of queuing commands without allocating
buffers for them. This way millions of commands could be queued on an
average Linux box without problems. Only ABORTs and they influence on
performance worry me.
As a data-point, some modern SCSI disks support queue depths in the
range of 128 to 256 commands.
I rather asked about practical upper limit. From our observations a
Linux initiator could easily send 128+ commands, but usually less. Looks
like it depends from its available memory. Interested to know the exact
rule.
- 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?
Well, it's the behavior implied by the SCSI standard; that is, on a
timeout, the initiator should abort the command. If an initiator sets
it's timeout to less than the queuing delay at the server, I wouldn't
call that "OK behavior", but it's not the target's fault, it's the
initiator's fault.
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?
I think it's up to the class driver to decide whether to retry a
command after it times-out.
Apparently, ABORTs must hit the performance at the similar degree as
too many QUEUE FULLs, if not more.
Much worse, I would think.
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?
I don't think you need to pick an unlimited size. Something on the
order of 128 to 512 commands should be sufficient. If you have multiple
logical units, you could probably combine them in a common pool and
somewhat reduce the number of command resources you allocate per
logical unit, on the theory that they'll not all be fully utilized at
the same time.
OK
By the way, make sure you don't deadlock trying to obtain command-
resources to return TASK_SET_FULL or BUSY to a command in the case
where the pool of command-resources is exhausted. This is one of the
tricky bits.
In our architecture there is no need to allocate any additional
resources to reply with TASK_SET_FULL or BUSY. So, we already took care
of this.
Thanks,
Vlad
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