Date: Mon, 11 Jun 2012 08:29:04 +0100 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: "Ronald F. Guilmette" <rfg@tristatelogic.com> Cc: freebsd-questions@freebsd.org Subject: Re: Making a bootable backup (hard)disk... how? Message-ID: <4FD59E40.3060506@infracaninophile.co.uk> In-Reply-To: <48302.1339366497@tristatelogic.com> References: <48302.1339366497@tristatelogic.com>
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On 10/06/2012 23:14, Ronald F. Guilmette wrote:
> Well, nevermind about that. I get the general idea, i.e. that dumping
> at level N causes dumping of everything that has changed since the last=
> dump at level N-1.
Not quite. A dump at level N includes everything that changed since the
most recent dump at level M where M<N.
So if you follow a level 0 dump immediately by a level 2 dump, the level
2 dump contains all the changes since the level 0 dump. Which is
exactly what you would expect.
Where this gets useful is if you consider what happens if the next dump
after that is a level 1 [+]. You get the difference between the
original level 0 dump and the current filesystem -- so you only need the
level 0 and level 1 tapes[*] to do a restore.
However if the third dump was a level 3, you'ld get the differences
between the level 2 dump and the current filesystem, so you'ld need all
of the level 0, level 2 and level 3 tapes to do a restore. It's a
toss-up between economising on the quantity of tape used and the hassle
factor in doing restores.
Cheers,
Matthew
[+] Actually level 2 or above. A dump level sequence of 0, 2, 2 has
the same effect as 0, 2, 1.
[*] For values of "tape" that include all sorts of different backup media=
=2E
--=20
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
JID: matthew@infracaninophile.co.uk Kent, CT11 9PW
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