[Linux-cluster] Snapshotting GFS and freezing

Pasi Kärkkäinen pasik at iki.fi
Mon Jan 18 10:34:39 UTC 2010

On Sun, Jan 17, 2010 at 02:47:00PM +0100, Dirk H. Schulz wrote:
> Ray Van Dolson schrieb:
> >>Hmm.. so snapshots with CLVM are possible nowadays?
> >
> >No....
> >
> >RH has stated (recently on this list) that patches exist to do it, but
> >it hasn't been a high enough priority for them to complete the work to
> >the point where it could be distributed to customers.
> >  
> Okay then, how do you people out there with clusters that run virtual 
> machines with live migration do backups of the virtual machines?
> I surely do not want to shut down the virtual machine to be able to copy 
> the image safely away if I have live migration available.
> At the moment there is only one way I can see. PLEASE prove me to be wrong.
> Searching in RedHat's documentation I found that the problem is that lvm 
> snapshots need exclusively allocated logical volumes. So I think the 
> following
> should be technically possible:
> 0. Start environment: the logical volume containing the images of the 
> virtual machines uses gfs and is mounted on all relevant cluster nodes 
> since VMs are running on several cluster nodes.
> 1. All VMs have to be migrated to one of the cluster nodes
> 2. On all other nodes, the gfs volume is unmounted
> 3. On the remaining node (where all VMs now run) the logical volume is 
> bound exclusively with "lvchange -aey LOGICALVOLUME"
>    (I hope this is possible without deactivating it first)
> 4. Now GFS on this volume is frozen: "gfs_tool freeze 
> /mountpoint/of/local/volume"

Before freezing the GFS you should make sure the VMs are in consistent
state, and the VMs have flushed their caches/buffers/disks.

> 5. Now the snapshot is generated and mounted
> 6. GFS is unfrozen again
> 7. The virtual machine images are copied off the snapshot
> 8. The snapshot is umounted und undone
> 9. The LV is activated on all cluster nodes again, GFS file system is 
> mounted on all relevant cluster nodes again.
> 10. The virtual machines are migrated back to whereever they belong.
> Now I would like to know:
> - is that basically the correct approach?
> - is there any better method to backup the virtual machines without 
> shutting them down?
> - is anyone out there using the above steps regularly in a production 
> environment?

-- Pasi

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