SMC-3 Read Element Status - Incomplete descriptors

[Elliott, Robert (Server Storage)]

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It would be more in line with other SCSI standards for the device server to
return partial descriptors.  The parameter data is supposed to be the same
(for a specific CDB) regardless of how many bytes are retrieved.
SPC-4's general convention for ALLOCATION LENGTH fields (spc4r06 4.3.4.6)
states that "If the information being transferred to the Data-In Buffer
includes fields containing counts of the number of bytes in some or all of
the data, then the contents of these fields shall not be altered to reflect
the truncation, if any, that results from an insufficient ALLOCATION LENGTH
value, unless the standard that describes the Data-In Buffer format states
otherwise."  The ALLOCATION LENGTH field could be anything from zero to
FF...FFh (based on the size of the field).
The STARTING ELEMENT ADDRESS and NUMBER OF ELEMENTS fields in the READ
ELEMENT STATUS CDB instruct the device server how many descriptors and which
descriptors to create.	The parameter data FIRST ELEMENT ADDRESS REPORTED,
NUMBER OF ELEMENTS AVAILABLE, and BYTE COUNT OF REPORT AVAILABLE fields are
all set by the device server without any regard to the allocation length -
those qualify as the "fields containing counts of the number of bytes".
Note 6 points out how the application client can figure out the best
ALLOCATION LENGTH to use - try 8 first to read just the header to examine
those fields.  Then, pick an allocation length big enough to retrieve the
data.  
-- 
Rob Elliott, elliott at hp.com 
Hewlett-Packard Industry Standard Server Storage Advanced Technology 
https://ecardfile.com/id/RobElliott 
  _____  
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Kevin D Butt
Sent: Tuesday, September 26, 2006 1:59 PM
To: t10 at t10.org
Subject: SMC-3 Read Element Status - Incomplete descriptors
SMC-3r02, 6.10.1 READ ELEMENT STATUS introduction, 2nd paragraph on page
321, reads: 
<< 
The NUMBER OF ELEMENTS field specifies the maximum number of element
descriptors to be created by the device server for this command. The value
specified by this field is not the range of element addresses to be
considered for reporting but rather the number of defined elements to
report. If the ALLOCATION LENGTH field is not sufficient to transfer all the
element descriptors, the device server shall transfer all those descriptors
whose complete contents fit within the allocation field and this shall not
be considered an error. 
>> 
There has been confusion by at least one vendor about the meaning of 
<< 
If the ALLOCATION LENGTH field is not sufficient to transfer all the element
descriptors, the device server shall transfer all those descriptors whose
complete contents fit within the allocation field and this shall not be
considered an error. 
>> 
It seems obvious that the intent is that only complete descriptors shall be
returned.  If the data is truncated due to an incomplete descriptor this
shall not be considered an error.  I propose that this text be modified to: 
If the ALLOCATION LENGTH field is not sufficient to transfer all the element
descriptors, the device server shall transfer all those descriptors that
completely fit within the allocation field and shall not transfer any
partial descriptors. This shall not be considered an error. 
Thanks, 
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Tel: 520-799-2869 / 520-799-5280
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at us.ibm.com
http://www-03.ibm.com/servers/storage/