Prioritized Unit Attention Reporting

Gerry.Houlder at Gerry.Houlder at
Fri Feb 11 06:52:33 PST 2005

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

Such a proposal is certainly within the scope of things the T10 committee
deals with. It is appropriate for the committee to consider it.

I suspect other members (including me) would want the priority to be
different for the items on your list, however.

Highest priority should be 06/29 - this should occur less frequently than
the others and it also has the highest impact on the target device in terms
of clearing commands, contingent allegiances, reservations, restoring saved
mode parameters, and possibly even resetting transfer rate agreements. For
some reason you seem to think this is the least important item to report
but I am sure others will share my opinion that this is the most important
event to report.

Likewise, an initiator is more likely to care more about having its
commands cleared by another initiator or losing its reservation (this
affects things the initiator is trying to do right now) than having a new
LUN become visible (this may influence things the initiator will do in the
near future but won't impact current work).

Be prepared to list reasons to justify why certain things are higher

Also be prepared to state why having each target figure out its own
priority scheme is not sufficient. If an initiator is going to be bombarded
with three unit attentions in a row on three consecutive commands, does it
really matter what order they are in? If the unit attentions don't really
occur at the same time, they will just be presented in the order they occur
regardless of the priority so the priority doesn't really matter for that

             peterson_gary at emc                                             
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                                       Prioritized Unit Attention          
             02/10/2005 03:50          Reporting                           

To:       T10

From:   Gary S. Peterson
            EMC Corporation, CLARiiON Division

Re:       Prioritized Unit Attention Reporting

Date:    February 10, 2005

            In the ongoing development of our storage systems, we have come
to the conclusion that reporting multiple Unit Attentions on an I_T_L nexus
basis is desirable to report the various conditions to the initiators
communicating with our array may encounter. However, our design does not
easily lend itself to open-ended queuing of Unit Attention conditions due
to the large number of I_T_L nexus that we support in our storage arrays.

            We are instead considering a "prioritized" Unit Attention
structure. Each potential Unit Attention report will be assigned a priority
level for reporting, with each level holding a single Unit Attention
condition. The highest priority Unit Attention condition will be reported
on the next command from the affected initiator. The second highest
priority Unit Attention condition will be reported on the second command
received from the affected initiator, and so on.

            The priority structure we're currently looking at that is along
the following lines:

            Priority 1 (Highest): Logical Unit Existence (as seen by the
initiator) Changed
                        06/3F/0E          REPORT LUNS DATA Has Changed

            Priority 2: Logical Unit Capacity Changed
                        06/3F/0A         VOLUME SET CREATED OR MODIFIED

            Priority 3: Logical Unit Access Changed
                        06/2A/04         RESERVATIONS RELEASED
                        06/2A/05         RESERVATIONS PREEEMPTED

            Priority 4: Logical Unit Task Set Changed
                        06/2F/00          COMMANDS CLEARED BY ANOTHER

            Priority 5 (Lowest): Logical Unit Parameters Changed
                        06/29/00          POWER ON, RESET OR BUS DEVICE
                        RESET OCCURRED
                        06/2A/01         MODE PARAMETERS CHANGED

A reading of the ANSI documentation regarding multiple Unit Attention
conditions only states that "Logical Unit may queue Unit Attention
conditions." (SAM-2, section 5.9.5).

      We therefore have two questions:

         1.      Is our proposed reporting scheme legal from an ANSI
         standards viewpoint?
         2.      Is this (or a similar) scheme something ANSI wishes to add
         to the standards?

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