[Linux-cluster] Snapshotting GFS and freezing

Pasi Kärkkäinen pasik at iki.fi
Wed Jan 20 19:15:48 UTC 2010

On Wed, Jan 20, 2010 at 03:14:02PM +0100, Dirk H. Schulz wrote:
> 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         sync
>        echo 3 > /proc/sys/vm/drop_caches
> inside the VM, right? Or is there anything more to do to flush everything?

And before those commands stop any databases or complex applications.. 
or make them flush their buffers and lock the tables for writes.

-- Pasi

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