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Re: [Linux-cluster] Snapshotting GFS and freezing



Pasi Kärkkäinen schrieb:
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.
Yes, of course, that is still another point in the list that has to be considered. But pausing your application (say, stop MySQL writing to the database) is not sufficient, you also have to write down buffers and caches of kernel, filesystem etc. to make the image as consistent as possible.

Dirk


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