Unit Attention and queued commands

Jeff Williams JWILLIAM at wdroc.wdc.com
Fri May 16 14:11:00 PDT 1997

* From the SCSI Reflector (scsi at symbios.com), posted by:
* Jeff Williams <JWILLIAM at wdroc.wdc.com>

The CA does not exist until the Check Condition is reported.
When the CA is created, if QErr = 0, the commands will be
frozen in the queue.  Any of those commands sent down could
have been used to indicate the Check Condition, but there
shouldn't be actions taken on behalf of the command which
reports the Check Condition.  The other commands will
complete normally once the CA is cleared.  The implication
in the statement you quoted is that when the command
completes with the CA for a Unit Attention, nothing was
done relating to that command.

As an aside, Charles is right in what will actually happen,
but there is no requirement that the first command reports
the CA (see queueing rules).


Jeffrey L. Williams                    Office: (507) 286-7589
Western Digital Rochester                 Fax: (507) 286-7528
Enterprise Storage Group       e-mail: jwilliam at wdroc.wdc.com

From:  Charles Monia[SMTP:Monia at mail.dec.com]
Sent:  Friday, May 16, 1997 2:49 PM
To:  'ljr at austin.ibm.com'
Cc:  'Reflector, SCSI'
Subject:  RE: Unit Attention and queued commands

* From the SCSI Reflector (scsi at symbios.com), posted by:
* Charles Monia <Monia at mail.dec.com>
ljr at austin.ibm.com[SMTP:"\"ljr at austin.ibm.com\""@SHDNS1.STARCH.MRGATE.SHR
Sent:  Thursday, May 15, 1997 6:43 PM
To:  monia at am.SHRMSG.SHR.mts.dec.com
Subject:  Unit Attention and queued commands

<<File: ATT18323.txt>>
* From the SCSI Reflector (scsi at symbios.com), posted by:
* ljr at austin.ibm.com (Les Record)

Assume we have a SCSI-2 target with QERR=0 (Mode page 0A), and we get
following sequence:

2   TAG 1 - COMMAND - disconnect
3   TAG 2 - COMMAND - disconnect
4   TAG 3 - COMMAND - disconnect
6   UNTAG - COMMAND=Request Sense - DATA=Sense Data - STATUS=Good
7   ????

What should happen to the commands queued prior to the CAC generation at
5, since they were received while a Unit Attention exists (due to the
but prior to the generation of the CAC?  The standard says (section 7.9,
paragraph 7):

"If an initiator issues a command other than INQUIRY or REQUEST SENSE
while a
unit attention condition exists for that initiator (prior to generating
contingent allegiance condition for the unit attention condition), the
shall not perform the command and shall report CHECK CONDITION

My opinion only:

The Logical Unit should have reported the pending UNIT ATTENTION by
returning a CHECK CONDITION on the command issued in step 2 after the
SCSI RESET.  Otherwise, as you imply, clearing the CA at step 6 will
allow commands issued in 2, 3 and 4 to complete without reporting the
CHECK CONDITION as specified in the paragraph you quote.

In other words, the scenario you describe should never have been allowed
to happen.


* Charles Monia                           *
* Storage Architecture Group              *
* Digital Equipment Corporation           *
* 334 South Street                        *
* Shrewsbury, MA 01545                    *
*                                         *
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