Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!mtxinu!rtech!gonzo!daveb
From: daveb@gonzo.UUCP (Dave Brower)
Newsgroups: comp.unix.questions
Subject: Re: backup with compressed cpio files ?
Message-ID: <516@gonzo.UUCP>
Date: 20 Feb 89 06:03:10 GMT
References: <9100001@netmuc> <872@deimos.cis.ksu.edu>
Reply-To: daveb@gonzo.UUCP (Dave Brower)
Organization: Gonzo Media Group
Lines: 42

In <872@deimos.cis.ksu.edu> tar@ksuvax1.cis.ksu.edu (Tim Ramsey) writes:
>In <9100001@netmuc> bothe@netmuc.UUCP writes:
>
>>has anyone tested to backup compressed cpiofiles ?
>>find . -print |  cpio -oac | compress >/dev/rmt0
>
>2) If you get a bad spot on the tape later you won't (never say never :-)
>   be able to recover anything beyond the bad spot.  This is a tradeoff
>   when you use compress.

Actually it's not hopeless.  I suffered a HD crash on my 3b1, and when
restoring my 60 compressed floppy backup found that disk #13 or so was
bad, keeping me from getting at anything beyond, at least so I thought
at first.

I contacted a few of the people whose names are in the comress source,
and they scratched their heads a bit and said essentially there are
three things you can do:

	1.  Hope the failure is deep enough that the tables are full
	    and stable and unchanging, so that you can just slice out
	    the bad section and have it uncompress OK.

	2.  Look past the bad section for some magic data that indicates
	    compress has decided the current table is trash, and is so
	    going to dump it and restart.  You could theoretically pick
	    it back up from that point.

	3.  Give up.

I was prepared to do #2, except I found after writing a program to 
reject damaged cpio archives (fixcpio) that I had only suffered #1.
So, it worked OK for me.

Now, given that many people will give up when their *uncompressed* cpio
archive has a bad spot, saying "out of phase -- get help", I don't 
think most people would lose anything by compressing.

-dB
-- 
"I came here for an argument." "Oh.  This is getting hit on the head"
{sun,mtxinu,amdahl,hoptoad}!rtech!gonzo!daveb	daveb@gonzo.uucp