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From: zwicky@erg.sri.com (Elizabeth Zwicky)
Newsgroups: comp.unix.admin
Subject: Re: SUMMARY: Backup while in multi-user mode
Message-ID: <1991May21.172208.281@erg.sri.com>
Date: 21 May 91 17:22:08 GMT
References: <1991May20.123129.14433@forwiss.uni-passau.de> <1991May20.204327.17694@erg.sri.com> <690@silence.princeton.nj.us>
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Organization: SRI International, Menlo Park, CA
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The "we'll just do live backups over and over again" theory suffers
from a common problem with security through redundancy; common mode
failures. All the backups may fail the same way if the same file is
always active when they're running. The easiest way to do this is to
accidentally get the backups synchronized with a cron job or a very
predictable human, but you can get the same effect with a very
long-running program. Thus, you can backup something a few hundred
times and have all the backups missing the same file. This is Not Fun.

Using tar instead of dump buys you extremely little. tar will skip
active files, which means they won't corrupt your backup. This is its
sole advantage, and its only an advantage over some versions of dump.
It will *also* skip files with names that are too long; depending on
the version of tar you are running, it may also exhibit various nasty
other problems dump doesn't have. On the whole, dump is safer.

	Elizabeth Zwicky
	zwicky@erg.sri.com