[Linux-cluster] Snapshotting GFS and freezing

Dirk H. Schulz dirk.schulz at kinzesberg.de
Sun Jan 17 13:47:00 UTC 2010

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 
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 
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 


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