[Linux-cluster] Snapshotting GFS and freezing

[Jeff Sturm]

> -----Original Message-----
> From: linux-cluster-bounces at redhat.com
[mailto:linux-cluster-bounces at redhat.com]
> On Behalf Of Dirk H. Schulz
> Sent: Friday, January 15, 2010 1:07 AM
> To: linux clustering
> Subject: Re: [Linux-cluster] Snapshotting GFS and freezing
> 
> > Or you can freeze, snapshot and unfreeze, and potentially be done
within
> > a few seconds.  That's the beauty of snapshots--we do it online with
> > little or no user disruption.
> >
> That means you can unfreeze gfs before undoing the snapshot? The
> snapshot has to be CREATED with frozen gfs but can go on WORKING with
> unfrozen gfs?

Snapshots are copy-on-write.  Once the snapshot is taken, further writes
to the origin device do not alter the snapshot.

> If that is right I would like to understand this in detail. Why does
gfs
> have to be frozen for creating a snapshot?

Good question--I've taken snapshots of ext3 volumes where no "freeze"
command is available.  It might have to do with flushing buffers or
playing journals.  Perhaps someone on the cluster list can explain
precisely what "gfs_tool freeze" accomplishes?