TASK SET FULL Definition(s) in FC-TAPE and SAM-2

Bob Snively Bob.Snively at Ebay.Sun.COM
Mon Mar 22 08:40:15 PST 1999

* From the T10 Reflector (t10 at symbios.com), posted by:
* Bob Snively <Bob.Snively at Ebay.Sun.COM>

George Ericson's note called to mind a recommendation we have made to
vendors of targets designed to support multiple initiators.  We strongly
recommend that a small number (2 or more) of "queue slots" be reserved
for each logical unit and for each initiator that the target has
identified.  In the case of FCP, this is simple, since each initiator
must log in with each target.  In the case of parallel SCSI, the
problem is a bit more challenging, but the number can either be
pre-specified, determined empirically by the target, or assumed to be
the entire device id address space.

The remaining queue resources, if any, should be put in a pool
available regardless of initiator.

By guaranteeing this, no initiator is locked out, since as commands
complete for that initiator, the queue slots dedicated to that
initiator remain available to it.  No initiator will get a queue full
indication with no outstanding commands.  At the same time, any
initiator with lots of work to do will obtain a relatively large number
of queue slots by competition for the pooled resources.

Using this approach, a relatively simple algorithm can be used by
device drivers to ratchet the assumed queue size up, while guaranteeing
that the initiator will never stall against the logical unit.
Completion of queued commands can be used as an indication that a
previously identified QUEUE FULL condition may no longer be present and
that the previously rejected command may be retried, since this will
eventually be guaranteed as the number of queued commands outstanding
dips below the guaranteed number of slots for that logical unit.

I assume that there are lots of variations that could be done on this
approach, including heuristic incrementing of guaranteed slots for
active initiators.

I see the BUSY state as something that a well-behaved target will rarely
offer, and then only for an unavoidable condition and a short time.
Longer term "busy" states should be indicated by the appropriate not-ready
check condition.



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