Compressed Backups

[Anthony DeBoer]

Awhile back there was some discussion of doing compressed backups, roughly
along the line of:

# find . -print | cpio -ovc | compress -v | dd bs=64k of=/dev/rmt0

At the time, there were some warnings posted that, with the usual compression
algorithm, a tape error would make the whole rest of the tape unusable since
uncompress would lose sync and the rest of the data stream would be garbage.

Now, it seems to me that the failure mode with every bad QIC tape I've ever
encountered has been that the whole rest of the tape was inaccessible anyway.
I'd like to inquire of the net: Have people in fact had tapes with errors or
other such glitches that they were in fact able to read past, and get at the
rest of the tape?  And does this have anything to do with QIC versus 9-track
versus other media?  It seems to me that with some of the low-level stuff
that happens on QIC, 9-track might be more amenable to recovery.  Can anyone
who knows low-level tape drivers comment?

What I'm getting at is, if in fact any tape error stops you dead anyway, then
there's no reason not to pipe a whole backup through compress (making sure you
have an uncompressed copy of uncompress available for recovery :-), since if
you can't get at the remaining data anyway it's academic whether you can
uncompress it.

Also, if you can back up the same data in half the tape, you can probably back
up the system twice as often and be better covered with the same volume of
tape.  In fact, you could show mathematically that you're just as well covered
this way with say a single-tape compressed backup daily as opposed to a
two-tape regular backup every other day, so long as the probability of the
compression itself preventing restore is less than 50% (the actual probability
would be considerably less!).  The exact ratio would vary with the "density"
of your disk contents, but we have some large data files that give better than
90% compression.

There's also the point that if your most recent backup was corrupt but allowed
you to continue and recover most of the system, you'd probably still go back
another generation if that restored perfectly, just so you'd have something
you trusted.  Since this is the one situation where compression would be an
issue, perhaps it wouldn't be after all.

I'm sure I don't need to request net.comment, but I will anyway.
-- 
Anthony DeBoer NAUI#Z8800 | adeboer at gjetor.geac.com   | Programmer (n): One who
Geac J&E Systems Ltd.     | uunet!geac!gjetor!adeboer | makes the lies the 
Toronto, Ontario, Canada  | #include <disclaimer.h>   | salesman told come true.