Querying Logical Units for Command/Task Queuing Depth

Gerry_Houlder at notes.seagate.com Gerry_Houlder at notes.seagate.com
Fri Jan 21 12:26:18 PST 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* Gerry_Houlder at notes.seagate.com
*
There isn't any workable technique to do this in a multi-initiator environment.
This is because the target can allocate resources to whatever initiator needs
them at the time. For example, a target may be capable of queuing 64 commands.
Initiator A may be able to send only one command or may be able to send 48
commands before receiving a QUEUE FULL indication. The variability of this limit
is dependent on how active other initiators are with that particular target.

If its a single initiator environment, the initiator will always be able to have
48 outstanding commands before receiving QUEUE FULL. In general, the target
doesn't really know how many initiators might want to talk with it. Fibre
Channel has a login procedure that helps with this knowledge, but a parallel
SCSI target must assume that 15 initiators might talk to it at any time.

Over the last few years various ideas of "early warning of queue full" or
"guaranteed number of queue slots per initiator" have been proposed and
discarded as unworkable (or not enough improvement to be worth the
complication). Most initiators have adopted vendor unique (and not necessarily
described to outsiders) methods for limiting the number of commands sent to a
target.





"Weber, William L" <William.Weber at UNISYS.com> on 01/21/2000 01:23:53 PM

To:   t10 at t10.org
cc:    (bcc: Gerry Houlder)

Subject:  Querying Logical Units for Command/Task Queuing Depth



* From the T10 Reflector (t10 at t10.org), posted by:
* "Weber, William L" <William.Weber at unisys.com>
*
Is there any SCSI command or method that can be used to query a SCSI-3
logical unit to report it's command queuing limits?  I know the standard
inquiry data will report CmdQue and BQue as defined in the SPC-2
specification. These flags will tell if command/task queuing is supported in
the logical unit but specifically I would like to know the command/task
limits before I get a "Task Set Full" status returned.

Regards,
// Bill Weber
// Unisys Corporation
// 2476 Swedesford Rd.
// Malvern, PA 19355-9700
// Email: 

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