Queuing and ACA

ROWEBER at acm.org ROWEBER at acm.org
Fri May 8 04:25:06 PDT 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* ROWEBER at acm.org
*
>I still do not quite understanding the underlying purpose of the ACA ACTIVE
>status, as opposed to simply letting new commands pile up in the blocked
>queue, or bouncing them with a simple BUSY status, since the initiator
>recovery action for ACA ACTIVE is exactly the same as for BUSY -- try again
>later.

Gerry Houlder did an excellent job of explaining why ACA ACTIVE is a
different status from BUSY.  My understanding of the reason commands
are not allowed to pile up in the blocked queue is related to protocols
such as Fibre Channel.

In a protocol such as FCP, a series of commands can be "in flight" to the 
target.  For performance reasons, the commands may have been put "in flight" 
in a way that has some of the later commands depending on the successful 
completion of some of the earlier commands.  I.E., assume all will succeed, 
and get those commands to the drive as quickly as possible.

If this model is followed (and there are those who call this practice
insane or worse), then the downstream "in flight" commands need to be
returned to the initiator for reconsideration whenever a command fails.
This is the reason for the ACA behavior.

Thanks.

Ralph...
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