Clearing effects of logouts on different protocols

Matthew Jacob mjacob at feral.com
Fri Mar 8 14:04:40 PST 2002


* From the T10 Reflector (t10 at t10.org), posted by:
* Matthew Jacob <mjacob at feral.com>
*


On Fri, 8 Mar 2002, Dave Peterson wrote:

> RE: Clearing effects of logouts on different protocolsKeep in mind that it
> is the OS tape driver/application that knows what going on. In practice,
> the OS tape driver/application will ensure the logical unit is in the
> proper state (from the drivers viewpoint) before reading or writing to the
> device. If there is anything that would cause an undetected change thus
> opening a data integrity hole, it needs to be fixed or at least
> understood.
> 
> What we need to do is to make sure the OS tape driver/application can
> continue to operate (within reason) across events e.g., reset conditions.
> If maintaining knowledge of the current partition is required for
> continued operation we should make every attempt to do so.

Yes, but the tape driver/application usually will make some pretty drastic
recovery choices after resets. After all, a tape device with removable media
is just about the most stateful device I can think of- after a reset (w/o
qualifying what 'reset' means here), you have *no* idea where you are on the
tape, or whether the tape is even the same tape any more.

You can recover from this, but usually it's a (literally) expensive
operation. If you're running something like NetWorker and were writing
to the tape, no matter where you were, you mark the tape as 'full' and load
another (via operator or from the changer). That's an expensive recovery from
'reset' in my book.

*If* the tape driver is aware that only something like operating parameters
were changed (and then can be restored), it *might* be safe to say that you
could assume that the tape logical position is where you expected it to be and
that the tape hasn't been changed. But that's a very big if.


-matt

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