userspace tool to freeze/thaw ext3 to create consistent snapshots

Jérôme Petazzoni jp at
Wed Sep 12 23:52:34 UTC 2007

Filip Sneppe wrote:
> Hi everyone,
> Suppose one has an ext3 filesystem on a SAN LUN.
> I would like to know if it is possible to freeze this ext3 filesystem
> into something
> consistent from a filesystem point of view, then trigger the SAN's snapshotting
> functionality, and then thaw the ext3 filesystem to resume I/O. This
> would allow
> near-instantaneous snapshot backups without having to use LVM or
> LVM snapshots.

I think (but I might be wrong) that this feature is not implemented -
yet - for ext3fs.
However, LVM2 should allow to create read-write snapshots.
Therefore, you can snapshot whenever you want, then mount the snapshot,
like you would mount an « unclean » filesystem (i.e. the log will be
replayed, and that's it).
One might argue that this could give unconsistent snapshots ; however
(the gurus will tell if I'm wrong) since your programs are running when
you do the snapshot, it should not change anything whether you do a «
hot » snapshot or a « thawed » snapshot.


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