RE: Command Queue Depth (was asymmetric/Symmetric)

[Robert Snively <rsnively@Brocade.COM>]

>  -----Original Message-----
>  From: julian_satran@il.ibm.com [mailto:julian_satran@il.ibm.com]
>  Sent: Thursday, September 07, 2000 6:11 AM
>  To: ips@ece.cmu.edu
>  Subject: RE: Command Queue Depth (was asymmetric/Symmetric)
>  
>
       snip
>  
>  As for SCSI queue length I assumed that the busy or queue full status
>  followed by an Asynch Event
>  message indicating readiness is the mechanism provided by 
>  SCSI to regulate
>  the command flow.

Actually, the approach was somewhat different, and is still only
a recommendation and not a requirement.

Every logical unit (not every target) was required to reserve
at least one command queuing buffer for each logged-in initiator
so that each logical unit could always accept at least one 
command.  Then the queue was stuffed full until TASK SET FULL
status was presented.  At that time, as each initiator encountered
TASK SET FULL, it would back off until a previously queued command
was completed.  At that time (serving the purpose of the
asynchronous event notification hypothesized above), the queue
could then be restuffed.  This back-off provided maximum queuing
for maximum throughput and allowed any particular initiator to
access all available resources at the time they were needed.  Given
the bursty (self-similar statistical) nature of storage accesses,
this appeared to be about the best we could do.  If a system ran
into continuing problems, additional resources (more devices, more
initiators, more links) were the only reasonable solution to 
maintain the desired level of performance.

The appropriate text from SAM-2 is provided here for your ASCII
reference.

Clause 5.2 Status

TASK SET FULL. This status shall be implemented if the logical unit 
supports the creation of tagged tasks (see 4.9). This status shall not be 
implemented if the logical unit does not support the creation of tagged 
tasks.

When the logical unit has at least one task in the task set for an 
initiator and a lack of task set resources prevents
entering a newly received tagged task from that initiator 
in the task set, TASK SET FULL shall be returned. When
the logical unit has no task in the task set for an initiator 
and a lack of task set resources prevents entering a newly
received tagged task from that initiator in the task set, 
BUSY should be returned.

When the logical unit has at least one task in the task set and 
a lack of task set resources prevents entering a
newly received untagged task in the task, BUSY should 
be returned.

The logical unit should allow at least one queued command for 
each supported initiator that has identified itself to
the target by a protocol specific procedure or by the successful 
transmission of a command.