[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: Thursday, January 14, 2010 12:21 PM
> To: linux clustering
> Subject: [Linux-cluster] Snapshotting GFS and freezing
> 
> Hi folks,
> 
> I found several howtos on the web stating that if you want to snapshot
a
> gfs volume (on top of clvm, of course), you have to freeze gfs (using
> gfs_tool) to make that possible.
> 
> What comes in mind then is: If I have to freeze the gfs volume anyway,
> do I need a snapshot at all? Can't I copy the contents off the frozen
> gfs volume directly?
> Because the basic purpose of a snapshot is to provide files that do
not
> change during beeing copied. And I would expect a frozen gfs volume to
> provide exactly that.

The important consideration here is:  How long can you withstand the
filesystem being frozen?

You can freeze it, copy all files (e.g. with rsync) and unfreeze,
potentially over the course of a few hours.  If that's okay with your
application and users, go right ahead.

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.

-Jeff