FCP_DL too small

Jim Jones (Engineering) Jim.Jones at quantum.com
Tue Nov 2 16:46:46 PST 2004


* From the T10 Reflector (t10 at t10.org), posted by:
* "Jim Jones (Engineering)" <Jim.Jones at Quantum.com>
*
Neil,

The original intent of this proposal was to post a check condition when the
FCP_DL was less than the transfer length contained in the CDB.  Here's the
background:

An application incorrectly specified an FCP_DL (48K) that was smaller than
the transfer length on a Write CDB (64K).  48K of data was written, and an
overrun was reported (FCP_RESID_OVER=1, FCP_RESID=16K).  But the application
also incorrectly only looked at the SCSI Status Byte and because it was Good
Status, continued on.  When the application tried to read the data, it
reported a data miscompare (because it didn't receive all the data that was
requested).  It took us a long time to sort all this out as an application
error.

I can't imagine a situation where it's valid for the FCP_DL to be less than
the transfer length contained in the CDB (in particular for DATA OUT
transfers).  For this reason we support this proposal.

This proposal wasn't meant to address any situations where the FCP_DL didn't
match the media record.

Hope this clarifies things.

Jim
__________________________
Jim Jones
Quantum Corporation
4001 Discovery Dr., Suite 2100
Boulder, CO 80303
Jim.Jones at quantum.com
720-406-5611
 
-----Original Message-----
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org]On Behalf Of Neil
Wanamaker
Sent: Monday, November 01, 2004 8:12 AM
To: t10 at t10.org
Subject: re: FCP_DL too small


* From the T10 Reflector (t10 at t10.org), posted by:
* "Neil Wanamaker" <neil.wanamaker at finisar.com>
*
03-393r3 proposes posting a check condition when FCP_DL is too small,
together with an indication that
there are illegal fields in the CDB.

This proposal makes no distinction between a mismatch between the
transfer length contained in the CDB 
and FCP_DL, and a mismatch between FCP_DL and the media record.

While the proposed change is not unreasonable in the first case, it
appears excessive in the second.

Since the days before there was a SCSI, a read of fewer bytes than the
tape blocksize is legal and results
in an indication of residual data. If  "the application client often is
not informed of the error", this seems to 
be a failing in the driver, rather than the architecture; complete
information is presented to the ULP.

Invoking a recovery procedure (as would be the result of a check
condition) seems inappropriate.


Neil Wanamaker
neil.wanamaker at finisar.com
512.670.7303 (O) 

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