Disks write-back cache behavior????

Bill Dallas dallas at zk3.dec.com
Thu Mar 30 03:48:53 PST 1995




I have a couple of questions about SCSI-2 disk drives write cache
behavior and was wondering if I could get a definitive answer here.  I
am asking theses questions here because I have talked to a number of
folks and everyone has a different opinion of what the behavior is for
write-back cache operation and resets.

Disk drive has write-back cache enabled and a bus reset occurs:
	For all completed (cmd comp. msg. sent ) I/O Processes that has
	write data in the cache that has not been placed on media.

	What happens to that write data, does it get flushed to the bit
	bucket, does it flush it to media or does it not do any special
	processing of that data and will get to media at some point in
	time?

SCSI-2 states:
	The effect of the reset condition on I/O processes which have
	not completed, .......

I can not find anywhere in the spec where a reset condition effects
completed I/O processes.

If the behavior of reset condition does not effect an completed I/O
processes and the associated write data for completed I/O processes is
flushed to media, what happens if:
	A media error is encountered:
		Is the media error reported as a deferred error along
		with the UNIT ATTENTION for the CA generated by the
		reset condition?

What about tapes that are running in buffered mode?  


Thank you from your time contemplating these questions.

Regards,
Bill Dallas




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