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Re: [Xen-users] Xen, LVM and snapshots

To: "Morten W. Petersen" <morten@xxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Xen, LVM and snapshots
From: Tomasz Chmielewski <mangoo@xxxxxxxx>
Date: Tue, 14 Oct 2008 23:09:57 +0200
Cc: Ferenc Wagner <wferi@xxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Morten W. Petersen schrieb:

(...)

Newer kernels (2.6.20, maybe even 2.6.19, and later) are not affected by
this "phenomenon".

Another LVM disadvantage is that every snapshot means additional writes.
I.e. if you have a logical volume, and 4 snapshots, writing to the
origin will mean 4 more writes are needed. This is a serious scalability
problem.

Well, what I was thinking, was to do a sync on the VM, and flush the
MySQL database and then immediately afterwards, taking a snapshot of the
system, backup that snapshot and then after backup "releasing"
(deleting) the snapshot.

We're stuck with .18 for now, as that's what Debian comes with and we
try to stick with off-the-shelf packages.

Do you think that's feasible?

Personally, I don't think taking snapshots is a good backup strategy, for several reasons:

- they take a lot of space
- the more snapshots you have, the more I/O load you will have (writes will go to every snapshot) - unless you're running a bleeding edge kernel with experimental patches, there is no way to turn the snapshot into original (other than copying it with dd, which will fill the snapshot, which means the snapshot has to be 100% of the size of the origin; which takes a lot of space) - for most Linux systems, taking backup is as easy as copying all files even from a live system; for databases, dump is of course needed to guarantee consistence; restoring backup is as easy as copying the files back - done - you really should make backups to a different machine, preferably somewhere else


That said, I use snapshots for:
- Windows - it's a pain in the a*** to restore a working Windows machine if you only have file backups; I usually keep snapshots for the last 3 days here (in addition to the usual file backup) - if I make any major changes, and don't have time to do a full file backup to another machine


For backup, I can recommend BackupPC.


--
Tomasz Chmielewski
http://wpkg.org

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