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From: les@chinet.chi.il.us (Leslie Mikesell)
Newsgroups: comp.unix.xenix.misc,comp.unix.xenix.sco
Subject: Re: Need assistance with afio problem.
Keywords: afio
Message-ID: <1991May29.160359.5889@chinet.chi.il.us>
Date: 29 May 91 16:03:59 GMT
References: <446@yonder.UUCP> <1991May26.203103.19212@chinacat.unicom.com> <447@yonder.UUCP>
Organization: Chinet - Chicago Public Access UNIX
Lines: 38

In article <447@yonder.UUCP> michael@yonder.UUCP (Michael E. Haws) writes:
>> >find /usr /u -print | afio -oZ -L/back/log | dd of=/dev/rct0 ibs=10b obs=1000b
[compressing with -oZ...]
>> If this is the case, methinks you
>> are making a *big* mistake.  For all intents and purposes, if one bit
>> is corrupt in an LZW compressed file, the following contents are
>> unrecoverable.  Using compress on backup volumes is penny-wise and
>> megatons-foolish.

>It is my impression that you are suggesting that I never keep compressed
>data on my hard disk, since this data would be subject to the same 
>potential problem when trying to recover it from a tape archive.  Or have
>I missed something?

The problem with compressing backups is that normally the entire stream
is compressed as a whole on its way to the archive device.  In that
case, a media error early in the tape will make it essentially impossible
to recover any data past that point (but some tape drivers won't continue
past a media error anyway, so in that case it wouldn't matter).  However,
this is a new option in a modified version of afio that was posted
to comp.sources.3b1 that compresses on a per-file basis by actually
making a temporary compressed copy on disk first.  This means that the
hunt-for-the-next-cpio-header method of error recovery will still work.
Other additions involve improvements for floppy access (make an in-core
copy to allow verify/rewrite per disk and ability to format a new
disk before continuing if there are problems). 

I'd dump the pipe to dd and let afio handle the device directly, though.
If you use the -f option it will go a little faster.  Also, I'd still
only use the -oZ if it's necessary to make a volume fit on a tape.
The compressed files won't automatically be uncompressed when you
restore and there is no way to tell which files were compressed on
the disk before the backup and which were compressed on the fly so
there would be a lot of work involved in getting a filesystem back
to its original state if you have to restore from one of those tapes.

Les Mikesell
  les@chinet.chi.il.us