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

[Kevin Fox <Kevin.Fox@xxxxxxx>]

If you were running a single OS with lvm snapshots (IE, no xen), would
the snapshots be consistent? IE, no fsck needed ever for the snapshots?
If so, then there must be a mechanism for LVM snapshotting to tell the
file system to coalesce to disk before the snapshot.

If there is such a connection between the LVM snapshotter and the file
system, I could see it not working properly if the LVM snapshotter is in
dom0 and the file system in domU. Perhaps a xen extension could be added
to support that?

Kevin

On Thu, 2008-10-16 at 19:12 -0700, Javier Guerra Giraldez wrote:
> 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
> 
> 
> plain text document attachment (ATT566064.txt), "ATT566064.txt"
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