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From: mangler@cit-vax.Caltech.Edu (System Mangler)
Newsgroups: comp.unix.wizards
Subject: Re: Backups on Live Systems
Message-ID: <2911@cit-vax.Caltech.Edu>
Date: Wed, 3-Jun-87 01:20:04 EDT
Article-I.D.: cit-vax.2911
Posted: Wed Jun  3 01:20:04 1987
Date-Received: Sat, 6-Jun-87 07:47:45 EDT
References: <132@dvm.UUCP>
Organization: California Institute of Technology
Lines: 41
Summary: 4.3 dump is more tolerant of active filesystems

In article <132@dvm.UUCP>, frank@dvm.UUCP (Frank Wortner) asks:
> How much am I risking if I try to dump a "live" filesystem?

Once I did some experiments where I ran dump while concurrently doing
an "rm -r" of most of the filesystem.

Restore would dump core pretty early on if I used 4.2bsd dump; but it
made it most or all of the way through if I used 4.3bsd dump.  And if
I didn't start the "rm -r" until after Pass I, II, and III of 4.3 dump
were safely out of the way, restore always made it all the way through.

4.3bsd dump has several checks in it to suppress things that are known
to cause core dumps in restore.  4.2bsd dump has no checks at all.
(4.3bsd dump also *outran* the rm -r).

I once changed dump to prompt for a tape between passes II and III,
so that passes I and II could proceed while the tape was being mounted.
This was a disaster, because the first tape was the operator's favorite
time to go to lunch, which stretched the critical section to an hour,
resulting in a rash of useless dumps until I figured it out.

If you have the choice, try to do full backups on quiesent filesystems,
and check them with fsck first.  A lot is riding on those full backups.

If you're stuck with dumping active filesystems,
    a)	Realize that the bigger the dump, the more likely it is to be
	bad, and the worse the consequences; thus, full dumps must be
	every week or two, to keep the probability of catastrophe down.
	Forget Towers of Hanoi; you need more redundancy than that.
    b)	Pick a time when not much is changing, like early morning.
    c)	Get through passes I, II, and III as fast as you can.  The time
	taken is linear in the number of inodes allocated, so it pays to
	allocate no more inodes than needed (also makes fsck go faster).

I've lost some data to bad dumps, but I've lost a lot more to dumps
that didn't get done.  It is probably better to do "live" dumps every
day than single-user dumps only weekly.  If you really care about the
risk of losing data, get Eagles, which are less likely to lose data
than most disk controllers.  (For this reason, I don't do disk-to-disk).

Don Speck   speck@vlsi.caltech.edu  {seismo,rutgers,ames}!cit-vax!speck