Re: [Xen-users] Backing up "live" Xen systems?

[Andy Smith <andy@xxxxxxxxxxxxxx>]

On Wed, Jan 04, 2006 at 04:59:48PM +0000, Steve Kemp wrote:
>   The only thing troubling me at the moment is backing up the
>  virtual instances.

[...]

>   Would it be possible to write a simple "fake" migration server 
>  to receive the image, write it to disk, and then "abort" gracefully?

As you've realised now, this will only get the memory.

I used to put everything on LVM and take snapshots, mount the
snapshot in dom0 and use my normal backup process to take care of
that.  However as the number of snapshots increased, I started
experiencing many lockups of the block devices I was snapshotting
(i.e. the domUs' filesystems).  If I was lucky I could clear them by
restarting domUs, but on a couple of occasions it required reboot of
dom0.

On reporting this to Red Hat it became clear that I wasn't the only
one, and Red Hat have confirmed that LVM snapshots are not stable
enough for production use.

Since then I have treated each domU as a separate server for backup
purposes.

>   That would allow a live system to be backed up without taking
>  it offline.

I do not think your pause/unpause or save/restore idea will be
acceptable as even an optimistic estimate would put the length of
time the domain must be paused for into the "several seconds,
possibly minutes" region.  That's enough to seriously affect network
traffic.

Note also that you have the general backup problem of not being able
to guarantee capture of your data in a consistent state, e.g.
databases.  That's always goin to be the case though, so rather
outside the scope of Xen.

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