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From: david@talgras.UUCP (David Hoopes)
Newsgroups: comp.unix.wizards
Subject: Re: Compressed Backups
Message-ID: <60@talgras.UUCP>
Date: 9 Apr 91 14:09:59 GMT
Article-I.D.: talgras.60
References: <1991Apr8.194026.29651@gjetor.geac.COM>
Reply-To: david@talgras.UUCP (David Hoopes)
Distribution: usa
Organization: Tallgrass Technologies Inc.
Lines: 56

In article <1991Apr8.194026.29651@gjetor.geac.COM> adeboer@gjetor.geac.COM (Anthony DeBoer) writes:
>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 think you are confusing a tape error (some section of tape is unreadable) 
with a user overwriting the beginning of tape.  Overwriting teh tape is
generally not recoverable on qic drives.  However  reading past a tape
error is posable.  It is more of a function of the software then the drive.
tar will choke and die.  Some newer versions of cpio will resync and 
continue the restore after a bad block.  You lose that file or even a 
couple of files but most of the backup is good.

I do not reccomend the above compression.  If you loose one byte off the
tape the rest of the tape is gone.

>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

I work with tapes and tape drivers.  I have induced tape errors with a magnate
on several QIC drives and our DAT drive.  It is possable to read past an
error if the software can handle 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

Try figuring out how much time this will add to doing your backup.  And don't
forget that the restore will take longer also.  You can do the same thing by
buying a couple of extra tapes.

>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.

What if you are just doing a selective restore?  Then if the error does not
fall in the files that you are restoreing who cares.


-- 
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David Hoopes                              Tallgrass Technologies Inc. 
uunet!talgras!david                       11100 W 82nd St.          
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