[linux-lvm] CLVM Snapshot HOWTO?

vu pham vu at sivell.com
Wed Mar 25 13:05:38 UTC 2009

Paul Dugas wrote:
> 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?

I believe that anything changed on the snap device won't affect the 
original device. The changes will go to the COW dev, which is created 
when you create the snap dev.


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