[Linux-cluster] Snapshotting GFS and freezing

[Pasi Kärkkäinen]

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