Re: [Xen-users] Xen backups using LVM Snapshots

[Javier Guerra Giraldez <javier@xxxxxxxxxxx>]

On Thursday 16 October 2008, Mike Lovell wrote:
> Javier Guerra wrote:
> > On Wed, Oct 15, 2008 at 2:17 AM,  <lists@xxxxxxxxxxxxx> wrote:
> >> Each DomU with a database has a cron that dumps the db with mysqldump. 
> >> Later on, Dom0 will do an LVM Snapshot of all DomUs and then do a file
> >> system backup of the Snapshots.  I do no actions on the DomUs.  No
> >> shutdown or 'xm pause/save/...'
> >
> > as you must have seen already, those snapshots mount as 'dirty'
> > filesystems, since they're exact copies of an already mounted
> > filesystem.  therefore, you have to run fsck on mount (usually
> > automatic on journalling filesystems).  depending on the journalling
> > algorithm, you risk having non-written data.
>
> This isn't sounding quite right to me. According to LVM Howto on
> tldp.org a snapshot is "exact copy of a logical volume, frozen at some
> point in time." (http://tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html).

right; but if that "point in time" was while it was mounted, the snapshot is 
exactly that: a non-unmounted volume.  just the same you would've got if you 
had pulled the plug on a running machine.  when you mount the snapshot, pay 
attention to warning messages, they'll tell the volume was dirty and run 
fsck.

-- 
Javier

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