Re: Questions on incremental backups

[Roman Mamedov <rm@romanrm.net>]

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On Fri, 18 Jul 2014 05:34:22 -0700
Duncan <1i5t5.duncan@cox.net> wrote:

> Effectively, admins can choose NOCOW XOR frequent-snapshotting, altho
> the fact that snapshots stop at subvolume borders can be used as a
> partial workaround, by putting NOCOW files on a dedicated partition and
> not snapshotting it, exactly as I mentioned.

You can't backup running VM images and datafiles of an active database using
"traditional" backup techniques such as file copy or rsync. The tail of a file
you're copying for a backup will be long-inconsistent with the overall state
or the head of the file when you started copying. Snapshots on the other hand
are atomic, and can very much be used to create a static copy of the files for
the purposes of compressing/copying away somewhere. And at worst, the
"restored from backup" state of such a backed up VM or DB will be equivalent
to it just having had a power-loss. Journalling FSes and databases can deal
with that with no major problems.

So just exercise moderation, snapshot e.g. once an hour or even a day, the
result will still be better than not using NOCOW, and will deliver most of the
benefits you get by snapshotting.

Another option is to snapshot->backup->delete snapshot.

-- 
With respect,
Roman

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