dump(8) verification -- really tape drives

[William LeFebvre]

> Note that a lot of this does not prevent what I have experienced as the
> absolute worst disaster in the whole backup industry: A tape drive is
> going out of synch, eventually someone notices something is wrong
> (maybe goes completely down), FS comes and fixes it. It now refuses to
> read many many tapes written on itself before the fix. This is not as
> uncommon as you think (have you ever checked after FS left?)

OH YES!!!  We got caught on this once.  And ever since then, whenever
FS does anything to the tape drive, I make sure BEFORE THEY LEAVE that
the drive can still read old tapes.  It's been our experience with
Digital and the TU-XX tape drives that it isn't that the heads were out
of alignment before, it's that FS engineer doesn't align them right!
And, take note, they test tape drives by running a diagnostic that
writes and then reads what it has written.  That is, it makes sure the
drive is consistent with itself, but not necessarily with the rest of
reality.

This also happened once with one of our RM-03 drives.  They replaced a
head that had crashed, aligned all the heads, declared it working, and
when I mounted a system pack, it couldn't boot off of it.  In this
case, tho, testing with an old pack was requested by the FS engineer.

In general, the responsible system manager should double check FS work
when necessary by trying old tapes and packs.  Granted, there are times
when it was wrong beforehand, and fixing it requires sacrificing the
ability to read old media.  But not usually.

			William LeFebvre
			Department of Computer Science
			Rice University
			<phil at Rice.arpa>
                        or, for the daring: <phil at Rice.edu>