Date: Thu, 28 Sep 2000 20:07:10 -0700 From: "Crist J . Clark" <cjclark@reflexnet.net> To: j mckitrick <jcm@FreeBSD-uk.eu.org> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: dump levels/incremental backups Message-ID: <20000928200709.J81242@149.211.6.64.reflexcom.com> In-Reply-To: <20000929033448.A59083@dogma.freebsd-uk.eu.org>; from jcm@FreeBSD-uk.eu.org on Fri, Sep 29, 2000 at 03:34:48AM %2B0100 References: <20000929033448.A59083@dogma.freebsd-uk.eu.org>
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On Fri, Sep 29, 2000 at 03:34:48AM +0100, j mckitrick wrote:
> I think I found enough scripts to help me choose a backup solution. But I
> have a couple of questions.
>
> Is there some secret to what the 'levels' are in dump?
dump(8) says,
-0-9 Dump levels. A level 0, full backup, guarantees the entire file
system is copied (but see also the -h option below). A level
number above 0, incremental backup, tells dump to copy all files
new or modified since the last dump of any lower level. The de-
fault level is 0.
Which means if you give a dump level of n, where n > 0, dump will
check /etc/dumpdates for the most recent dump of at a level less than
n. All files modified since that date are dumped.
> What is the purpose of the 'Towers of Hanoi' number pattern?
It is a trade between backup size and the number of restores that will
need to be done to any given point. The series in the manpage is,
3 2 5 4 7 6 9 8 9 9 ...
For size concerns, notice that each dump contains no more than two
steps worth of changes. Now when we need to restore, say we have a
problem at,
3 2 5 4 7 6
This point of the cycle. To restore the filesystem, we start with the
level 0, then 1, then 2, then 4, and then 6.
For a comparison, let's say we do a full dump, the "weekly" level 1,
then just a level 9 for all intermediate steps,
9 9 9 9 9 9 9 9 9 9 ...
Now, if we ever had to restore, we only need do, level 0, 1, then
9. However, note that the size of the dumps is increasing
monotonically. That is, every file that was modified on that first
step backup is _also_ backed up every step after (unless it got
deleted).
Let's look at the other extreme,
2 3 4 5 6 7 8 9 ...
Now the size of each dump is minimized, but restoring is now gonna be
a real hassle since you need to use every dump.
> Is the tar incremental backup simply based on comparing the modification
> times of the existing files to the stored ones?
Don't use tar for backups, but I would assume so. What else would you
do?
--
Crist J. Clark cjclark@alum.mit.edu
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