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Re: [Xen-users] Backup domU

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Subject: Re: [Xen-users] Backup domU
From: Jonathan Tripathy <jonnyt@xxxxxxxxxxx>
Date: Sat, 09 Jul 2011 22:08:04 +0100
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On 09/07/2011 21:58, Steve Allison wrote:
On 09/07/2011 10:42, Simon Hobson wrote:
Steve Allison wrote:

The backup is also on the same disks as the live system.

I don't know why you make this point, as, at least in this thread,
the target of the backup is not mentioned and is trivial to have it
point to another system.

I wasn't aware LVM could create a snapshot which puts a complete copy
of the volume on another disk. In fact, AUIU, a snapshot doesn't
actually copy the whole volume, it creates a COW setup which saves any
blocks subsequently written to.

Exactly. So you put the LVM into a snapshot state, and "backup off the
snapshot, nice and clean then destroy the snapshot when finished".

To be honest, IMO, I think the best method is a mixed solution:

1) Create snapshot-based backups as described above (using rsync, rsnapshot, dd, cp, or whatever). This will take care of most files OK, and will certainly allow for a DomU to be booted up ok in the event of a system crash.


2) have a seperate solution for applications that require your filesystem to be in a consistent state - Simply making a snapshot as in 1) doesn't provide this guarantee. Good options here are the tools that come with most good database applications (pgdump for PostgreSQL is an easy choice).

By creating snapshot backups alone (as in 1)), unless your DomU has been cleanly shutdown beforehand, will never guarantee a consistent filesystem. Expect to run "e2fsck -f" before using such a backup again.

At least that's my grasp on the subject...

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