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From: jay@silence.princeton.nj.us (Jay Plett)
Newsgroups: comp.unix.admin
Subject: Re: SUMMARY: Backup while in multi-user mode
Summary: redundancy pays
Message-ID: <690@silence.princeton.nj.us>
Date: 21 May 91 07:30:05 GMT
References: <1991May20.123129.14433@forwiss.uni-passau.de> <1991May20.204327.17694@erg.sri.com>
Organization: Access, Inc.
Lines: 39

In article <1991May20.204327.17694@erg.sri.com>, zwicky@erg.sri.com (Elizabeth Zwicky) writes:
> In article <1991May20.123129.14433@forwiss.uni-passau.de> baier@unipas.fmi.uni-passau.de (Joern Baier) writes:
> >Nearly everyone who answered pointed out that at his site the backups are
> >run while in multi-user mode  but nobody has already observed a serious error
> >as a result of this policy.
 ...
> Yes, it does cause problems, I have seen it do so ...
> ... Most people do very few restores; many of the people who have
> never had a problem have never done a full restore, either. 
I have done full restores.  Not a lot of them, relative to the number
of dumps.  I have never had a problem restoring a dump made on a live
filesystem.  This does not imply that I never will.

> ... If you cannot risk
> having a backup be bad, don't do it in multi-user. 
Good advice.

> You can probably
> risk having your daily backups be bad.
Ah, there's the point.  If you can _risk_ losing one or two days work,
then do daily level 0s on live filesystems.  This is the beauty of
Exabytes--it is feasible to do so.  If a tape is bad at restore time,
toss it and go back a day.  If that one is bad, go back another day.
The risk dimishes greatly with each day you go back.

Look at the odds.  The probability of a disk crash on any particular
day is really very small.  The probability of a bad level 0 done on a
live filesystem might be larger, but it's still small.  The probability
of two successive bad tapes is smaller.  Apply your favorite function
to calculate the probability of a bad tape coinciding with both of the
two days before a crash, and decide if that risk is acceptable to your
users.  Balance the risk against the cost to your users of routinely
shutting down for backups.  Don't forget to evaluate the possibility
that a dump of an idle system might also be unrestorable.

I believe that--for many sites--the advantages of dumping live filesystems
outweigh the disadvantages.

	...jay