Unit Attention and Reservation Conflict

ljr at austin.ibm.com ljr at austin.ibm.com
Fri Oct 6 05:56:52 PDT 1995

I have experienced two different responses to this situation, from two 
different device manufacturers:

Situation A: While reserved by initiator A, the device returns CHECK CONDITION 
to initiator B's TUR with a subsequent sense key of Unit Attention on 
initiator B's Request Sense, then returns RESERVATION CONFLICT to future
commands (including TUR) from initiator B.  After initiator A releases the
device, Unit Attention remains cleared for initiator B.

Situation B: While reserved by initiator A, the device returns RESERVATION
CONFLICT to all TUR commands from initiator B, then returns CHECK CONDITION
(for a subsequent sense key of Unit Attention) to initiator B after initiator A
releases the reservation.

Situation B is the correct response.  Although a Request Sense command from 
initiator B can be processed by a device that is reserved by initiator A, the
Test Unit Ready command cannot (see standard section and should have 
received RESERVATION CONFLICT instead.  In situation B, since the Contingent 
Allegiance condition was not established (see standard section 7.6 - a CAC does
not exist until AFTER the return of the CHECK CONDITION to the initiator), Unit
Attention could not be cleared for that initiator until after the reservation 
was released.

Part of the confusion arises in the discussion of Unit Attention condition in 
section 7.9.  The second-to-last paragraph makes reference to the priority of 
status conditions AS DEFINED BY THE TARGET.  If an implementor considers a 
CHECK CONDITION to be higher priority than RESERVATION CONFLICT, then situation
A might arise.  Nowhere in the standard are explicit recommendations or 
guidelines as to what the proper priority is, but it is implied (from an 
analysis like this) that RESERVATION CONFLICT is the higher priority.

Les Record
ljr at austin.ibm.com
(512) 838-3622
Austin, Texas
----------------------------- Note follows -----------------------------
From: sgaskill at qntm.com
Date: Thu, 5 Oct 1995 10:58:13 -0700
Subject: Unit Attention and Reservation Conflict

Based on section 7.9 in SCSI-2 rev 10K (second half of section) it is unclear as
to what action the target should take in the following situation.

1. Target has a pending Unit Attention condition for all initiators.
2. Initiator A selects the target, gets the Unit Attn, and then creates a 
"normal" reservation (not 3rd party, not "persistent").
3. Initiator B issues a Test Unit Ready to the target and gets the Reservation 
Conflict (higher priority status as noted in 7.9).
4. Initiator A releases the reservation.
5. Initiator B issues another Test Unit Ready to the target.

Q: Will Initiator B get a Unit Attention OR should the UA condition have been 
cleared by the TUR which resulted in a reservation conflict?

Alternate question:  What do most peripherals do in this situation?

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