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

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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 
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.
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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