[Linux-cluster] Snapshotting GFS and freezing

[Pasi Kärkkäinen]

On Sat, Jan 23, 2010 at 11:12:54AM +0100, Dirk H. Schulz wrote:
> Ray Van Dolson schrieb:
>> On Wed, Jan 20, 2010 at 06:14:02AM -0800, Dirk H. Schulz wrote:
>>   
>>> 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?
>>>
>>> Dirk
>>>     
>>
>> I would think the best way would be to actually pause the VM.  If you
>> snapshot a VM while it's running, even with the above, how can you
>> guarantee an application won't do something on the VM right as the
>> snapshot occurs?
>>   
>
> That is an important point, I am already pondering about it. Of course  
> pausing the vm is the better option - if you can afford it.
>
> I am working systematically on an environment where the possibility of  
> very high level availability vms is part of the design (e.g. 99,99 %).  
> At the moment the only approach I can see is doing snapshots of a  
> running vm and making those as reliable as possible.
>

Pausing the VM doesn't really help. It doesn't guarangee in any way that 
the applications (say MySQL) are in a safe/consistent state.

You really need to coordinate the process with the applications, if you 
realy need/want to make consistent backups with snapshots.

-- Pasi