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.
Regards,
-Steve
--
Steve Byan <smb@xxxxxxxxxxx>
Software Architect
Egenera, Inc.
165 Forest Street
Marlboro, MA 01752
(508) 858-3125
-
: 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
