[linux-lvm] CLVM Snapshot HOWTO?

Paul Dugas Paul at dugas.cc
Wed Mar 25 04:15:29 UTC 2009

On Mon, 2009-03-23 at 22:34 -0500, Vu Pham wrote:
> Paul Dugas wrote:
> > I apologize if this has been covered already or is written up elsewhere
> > but I can't seem to find it.  I've got a couple of AoE volumes that I'm
> > using for LVM VGs.  They have GFS filesystems on LVs that are sized to
> > fill 90% of the the VGs; 10% space left over for snapshots.  I have 4
> > CENTOS 5.2 machines clustered and mounting the GFS filesystems.  That's
> > working well.
> > 
> > Having used LVM snapshots in non-cluster environments before, I figured
> > I'd just use the same logic for backing up the volumes; lvcreate -s,
> > mount, backup, umount, lvremove.  This quickly fell apart and I started
> > to think about it more.  I ended up wondering how snapshots of the
> > volume could ever really work without coordinating with the other
> > nodes.  
> > 
> > My question is this.  Should I be able to use snapshots on clustered
> > volumes like this?  If not, are there plans to support it later?  If so,
> > can someone point me to a working example?
> > 
> I think you have to freeze the GFS volume before you create a snapshot, 
> then you unfreeze it. man gfs_tool

That's what I was looking for.  Much thanks for getting me further but I
think I'm still missing a step.  Currently, this is what my "pre-backup"
script is doing:

        COOKIE=`gfs_tool list | egrep " $LK_TBL_NM.[0-9][0-9]*\$" | awk '{print \$1}'`
        gfs_tool freeze $COOKIE
        lvcreate --size ${SIZE} --snapshot --name SNAP /dev/${VG}/${LV}
        gfs_tool unfreeze $COOKIE
        gfs_fsck -y /dev/${VG}/SNAP
        mount -t gfs -o ro,lockproto=lock_nolock /dev/${VG}/SNAP ${MNTPT}

I'm getting a "File Exists" error from the mount which I believe is
because I'm trying to mount both the device and the snapshot at the same
time and they share the same lock-table-name.   Now that I think of it,
I don't really need to mount the original version of the file system so
I may be able to move past this hitch but I'm wondering if there's a
good way to address this?  Could I rename the lock table via "gfs_tool
sb /dev/${VG}/$SNAP table $LK_TBL_NM.SNAP" to get around it?  Would that
change be cleared when the snapshot is removed?

Paul Dugas - paul at dugas.cc - 404.932.1355
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