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Re: [Xen-users] Backup; mount domU FS RO?

To: Ralf Folkerts <ralf.folkerts@xxxxxx>
Subject: Re: [Xen-users] Backup; mount domU FS RO?
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Date: Mon, 19 Dec 2005 08:04:50 +0100
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On Sun, Dec 18, 2005 at 09:26:12PM +0100, Ralf Folkerts wrote:
> Hi,
> I recently set-up a Xen Server that's hosting 3 Development Systems. It
> works quite well, so it's time to put this into Production -- and to
> perform regular Backups.
> We use Bacula to Backup the dom0. My Idea was, to mount all domU
> Filesystems Read-Only to some Directories and then back these up from
> the Bacula-FD on the dom0.
> I found some warnings that Filesystems may not be mounted RW by more
> than one domain / system (makes sense when they don't "know" about each
> other). However, are there any issues mounting a Filesystem Read-Only,
> then have it being backed-up by Bacula and then unmount it?

Very bad idea. If you mount the FS read-only, the kernel will assume
that certain things won't change and it is going to get some nasty
surprises when they _do_ change.

In short: Don't do this.

A better approach is to use LVM and snapshots. Use LVM logical volumes
as "disks" for your DomU (if you aren't already doing this), create
snapshots for your backups, mount the snapshots and backup _those_.
After the backup, umount the snapshots and destroy them.
Don't forget to allocate enough storage to your snapshots to cover any
changes in the volume.

I'm currently using this to backup our production DomU systems from the
Dom0 and it works very well.

"Opportunity is missed by most people because it is dressed in overalls and
 looks like work."                                      -- Thomas A. Edison

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