Objections to 10-043r0 changing the intent of the wtc bit in WRITE ATTRIBUTE command
Kevin D Butt
kdbutt at us.ibm.com
Fri Mar 5 14:40:29 PST 2010
Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1003052_f.htm">HTML-formatted message</a>
Curtis,
Regarding your proposal 10-043r0 "SPC-4: Correct usage of term volume in
MAM attributes" I have objections with the proposed modification of the
wtc bit.
The wtc (write-through cache) bit which was recently included in SPC-4 (
see 09-355r2) is intended to provide a blocking synchronization to MAM
much like a WRITE FILEMARK is supposed to provide a blocking
synchronization of buffered data to medium. The application is blocked
until GOOD status is returned and then the application can be assured that
the data is out of the buffer and onto the medium. That is the same
intent as the WTC bit with data going to the MAM.
It would not make sense for a device server to return ILLEGAL REQUEST,
OPERATION IN PROGRESS to a WRITE FILEMARK command even if the drive was
doing some extensive error recovery procedure like a head brush that takes
a long time. The WRITE FILEMARK command is held in the queue and waits
until it can be serviced or until the upper layer timeout in the
application fires and the command is aborted. In the same token, it does
not make sense for a WRITE ATTRIBUTE command with the WTC bit set to one
to return this error just because a device server is busy with something
else. The WRITE ATTRIBUTE command should be held in the queue until it can
be serviced and return GOOD status.
Additionally, should an application receive a response of ILLEGAL
REQUEST, OPERATION IN PROGRESS to the WRITE ATTRIBUTE command what would
be its actions? It doesn't know why this is returned. It doesn't know if
it should reissue the command and if so how long to wait. In practice
applications would fail the entire job, call the volume bad, and move to
use a different volume. This is not desirable.
Thanks,
Kevin D. Butt
SCSI & Fibre Channel Architect, Tape Firmware
MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
Tel: 520-799-5280
Fax: 520-799-2723 (T/L:321)
Email address: kdbutt at us.ibm.com
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