SSC-2 Action Items

Gerry.Houlder at Gerry.Houlder at
Tue Sep 4 11:39:27 PDT 2001

* From the T10 Reflector (t10 at, posted by:
* Gerry.Houlder at

I typically don't attend SSC meetings, but I felt the need to comment on
the first agenda item.

Guidance for "what should be done if command queuing is not supported"
should be the same as if the DQue bit in mode page Ah is set. This control
bit has been around for a long time but may not be used by many systems.
Setting this bit is a good way to indicate that command queuing is NOT
supported. If the bit is not changeable, it is a good indication that the
host should not ever expect to use the "other" setting of the bit.

When DQue bit is set (queuing is disabled), past products will respond with
"Message Reject" message when any of the two byte queue tag messages is
received. The Message Reject message should be sent after the second byte
of the two byte message. After sending the message reject, most targets
will continue on to command phase and process the remaining phases as if
the command is an untagged command. This could mean that the target will
see the command as overlapping a previous untagged command and aborting
both commands per the "overlapped command" rules.

The "continuing on after Message Reject" response is not required anywhere
but is considered the normal way to proceed after a Message Reject case.
Another possible response is to go unexpected bus free after sending
Message Reject message. This has a slight advantage over the previous case
in that a previously accepted untagged command will not get aborted. The
disadvantage is the total lack of system reporting about why the command
didn't work.

Another possible response is for the target to return Message Reject
message after the queue tag, then go to status phase and report CHECK
CONDITION status. The subsequent sense bytes can report "illegal message
received" sense bytes. This has the advantage of having the best system
error reporting at the cost of being a little more complicated.

I hope your group will choose its response from this limited group of
"considered acceptable" responses rather than creating a new response.
Remember, the only thing worse than not being able to do something is to
have 4 ways of doing something and having to handle all 4 ways!

"Dave Peterson" <dap at> on 09/04/2001 11:43:44 AM

Sent by:  owner-t10 at

To:   "T10 Reflector" <t10 at>

Subject:  SSC-2 Action Items

* From the T10 Reflector (t10 at, posted by:
* "Dave Peterson" <dap at>
Couple of "All" action items for the SSC-2 Working Group:

1. Determine what is currently done and what should be done if a target
does not support command queuing receives a tagged command.

2. Determine how drives which do not support unloading at EOT will behave
when LOAD UNLOAD has LOAD=0 and EOT=1.


David A. Peterson
Cisco Systems, Inc.
Lead Architect - Standards Development
6450 Wedgwood Road
Maple Grove, MN 55311
Email: dap at
Cell: 612-802-3299
Office: 763-398-1007

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