[Linux-cluster] Snapshotting GFS and freezing

Dirk H. Schulz dirk.schulz at kinzesberg.de
Wed Jan 20 14:14:02 UTC 2010

Pasi Kärkkäinen schrieb:
> 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.
That means calling  
        echo 3 > /proc/sys/vm/drop_caches
inside the VM, right? Or is there anything more to do to flush everything?


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