Path: utzoo!mnetor!uunet!husc6!rutgers!ames!sdcsvax!ucsdhub!jack!crash!ford
From: ford@crash.cts.com (Michael Ditto)
Newsgroups: comp.unix.wizards
Subject: Re: Tar with multiple volumes
Message-ID: <2212@crash.cts.com>
Date: 27 Dec 87 20:29:14 GMT
References: <341@tness1.UUCP> <19797@teknowledge-vaxc.ARPA>
Reply-To: ford%kenobi@crash.CTS.COM (Michael Ditto)
Organization: Crash TS, El Cajon, CA
Lines: 52
Summary: Make each volume independantly usable, or just use a shell script

In article <341@tness1.UUCP>, mechcrm@tness1.UUCP (Chet Murthy 713+680-8841) says:
> Hi.  I am in the process of writing a multiple-volume tar/cpio/backup
> utility...
> 
> What exactly would be useful in a multivolume tape backup utility, assuming
> it was based on tar?

It must make each volume (disk, tape, or whatever) usable even if others
in the set are lost.  In other words, the obvious technique of simply
splitting the archive into N-block chunks (where N is the size of a disk
or tape) is inconvenient, because if any volume is lost, all volumes after
it are essentially useless.  If this simple solution is sufficient,
however, it can be implemented with a shell script.

For example, (from memory, this probably wont work exactly as is):

	# "mdtarout" (multiple disk tar output) assumes a disk is 800 blocks.
	tar -cvf - "$@" |
 	(
 		while echo 'Insert the next disk and press return\c' >/dev/tty
			read x < /dev/tty
		do
			dd count=800 > /dev/floppy || exit
		done
	)
	exit


	# "mdtarin" (multiple disk tar input) assumes a disk is 800 blocks.
 	(
 		while echo 'Insert the next disk and press return\c' >/dev/tty
			read x </dev/tty
		do
			dd count=800 < /dev/floppy || exit
		done
	) |
	tar -xvf - "$@"
	exit

I use something like the above for making backups which I don't expect to
use unless something catastrophic happens, or for transferring a large
software package that would be useful if extracted as a whole.  The only
reason to write a multi-volume tar would be to make it better than the above
shell scripts can be; i.e. to split the volumes at file boundries, and such.
Even that can be done with existing tools like awk.  If you write such a
program it had better be loaded with user-friendly niceties to make it
worth using.
-- 

Mike Ditto					-=] Ford [=-
P.O. Box 1721					ford%kenobi@crash.CTS.COM
Bonita, CA 92002				ford@crash.CTS.COM