SPC-3 revision 12 REQUEST SENSE 18 byte rule change

Elliott, Robert (Server Storage) Elliott at hp.com
Thu Mar 20 17:04:15 PST 2003


* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert (Server Storage)" <Elliott at hp.com>
*
One of the changes to REQUEST SENSE in SPC-3 revision 12 might have
unintentionally 
changed the meaning.

SPC-3 revision 11:
Device servers shall be capable of returning eighteen bytes of data in
response
to a REQUEST SENSE command. If the allocation length is eighteen or
greater, and
a device server returns less than eighteen bytes of data, the
application client
should assume that the bytes not transferred would have been zeros had
the device 
server returned those bytes. 

SPC-3 revision 12:
Device servers shall return at least eighteen bytes of data in response
to a
REQUEST SENSE command if the allocation length is eighteen or greater.

These sentences remained unchanged:
Application clients may determine how much sense data has been returned
by
examining the ALLOCATION LENGTH field in the CDB and the ADDITIONAL
SENSE LENGTH 
field in the sense data. Device servers shall not adjust the additional
sense 
length to reflect truncation if the allocation length is less than the
sense 
data available.


The 18-byte references were based on the fixed sense data format.  With
the 
descriptor sense data format, the minimum size is 8 bytes, not 18 bytes.

The application client is likely to use an ALLOCATION LENGTH of 252
bytes since it 
doesn't know the size of the sense data.  Why should the device server
include 10 
extra bytes if the descriptor format only needs the minimum size?  What
would those 
extra 10 bytes be set to?

I suggest just including this warning for application clients about old
targets,
and remaining silent about the device server:

If fixed format sense data is requested, the allocation length is
eighteen or greater,
and a device server returns less than eighteen bytes of data, the
application client
should assume that the bytes not transferred would have been zeros had
the device 
server returned those bytes. 

--
Rob Elliott, elliott at hp.com
Hewlett-Packard Industry Standard Server Storage Advanced Technology
https://ecardfile.com/id/RobElliott



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