IBM's response to 12-405r0 SAM-5: UA and task set interaction claraification

Kevin D Butt kdbutt at
Mon Oct 15 16:47:18 PDT 2012

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IBM has comments related to 12-405r0
SAM-5: UA and task set interaction claraification
(by: Frederick Knight)
T10/12-405r0   Uploaded: 2012/10/08   175917 bytes
The current SCSI standard can be interpreted in multiple ways for UA 
29/00. IBM would agree that clarification might be useful here providing 
we can agree on what that should be. However IBM does not think that 
clarification should be the current proposal in 12-405r0. 
Under this proposal if a lun has a UA 29/00 pending, multiple tasks would 
be allowed to be accepted by that lun. The first task processed (enabled) 
would fail with UA 29/00. The subsequent ones would be governed by QErr 
bit settings. This seems problematic for a number of situations where a 
device's state may have substantially changed thus compromising data from 
the initiator perspective. Thus the subsequent tasks may be processed from 
a device that is in a questionable state (i.e. loss of reservations and 
thus no synchronization of read/writes with other hosts, difference in 
mode settings etc). 
A way to avoid the above issue would be to have the device support QErr = 
x1 settings. This though is problematic, since a device may not support 
this setting. 
A more robust solution (from the initiator's perspective) is for the first 
command to be failed immediately with UA 29/00 before other commands are 
accepted. The subsequent commands would then be governed by whether the 
faulted command had the NACA=1 bit set. If so then subsequent commands 
would be failed with ACA ACTIVE status. This is how some of IBM's 
engineers interpret the standard. Based on how SCSI evolved, IBM thinks 
that this is more consistent with the original intent of the standard. 
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
Data Protection & Retention
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Tel: 520-799-5280
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at

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