Disks write-back cache b

Jim McGrath jmcgrath at qntm.com
Thu Mar 30 09:44:47 PST 1995


        Reply to:   RE>Disks write-back cache behavior????



Bill,

I can answer the hard disk drive questions:

 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?

We (Quantum) have always stated that once we cache data it will be written
to the media barring a fatal drive problem (i.e. head crash) or power
loss.  Specifically, we take care to execute the reset on the bus while
continuing to keep track of the cached data and writing it back to the 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?

We do not report deferred errors in general.  We found that customer drivers
did not expect or want this.  Instead, we use the most extensive error
recovery
process possible in the drive and back it up with automatic reallocation of
the
error to a new portion of the media.  The only drive failure mode that can
defeat us is the inability to write anywhere for seconds at a time - true in
a fatal drive failure (i.e. head crash), but very hard to induce otherwise
(e.g. shock and vibration could do it, but it is very hard to defeat the drive
for all those retries without also violating the drive shock and vibration
specification - we have never seen such a case in practice).

Given this, there is no real use in reporting a deffered error, since there is
no error recovery operation the system could perform on that drive that
we have mot already attempted several times.  After all of this work, any
residue probability of an error is far lower than the probability that we
simply would write the data incorrectly to the media (something that you
cannot always detect unless you also do a verify of the media after it is
written) - such events are the main contributer to the drive's hard error
rate.

Jim







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