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Date: Thu, 10 Mar 2011 10:32:36 +0100
From: Jes Sorensen <Jes.Sorensen@redhat.com>
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Subject: Re: [Qemu-devel] Issue with snapshot outside qcow2 disk - qemu 0.14.0
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: SAURAV LAHIRI <saurav_lahiri@yahoo.com>, qemu-devel@nongnu.org

On 03/10/11 10:27, Stefan Hajnoczi wrote:
>> Is Scenario 2 safe where in the copying the snapshot outside the original qcow2 is being executed with the VM running. This is because if this is safe then this could be an approach as it would not require a long downtime for the VM.
> 
> There is no guarantee that qemu-img will work on an image file that is
> open by a running VM.

I think the guarantee here is that you're guaranteed it will go horribly
wrong if you try to do so.....

> I have CCed Jes who has been working on a live snapshot mechanism.  He
> recently added the snapshot_blkdev monitor command that takes a
> snapshot of a block device while the VM is running.  A new image file
> is created based off the original image file (which will no longer be
> modified), all new disk writes go to the new image file.  It is safe
> to perform read-only access to the original image file.  There
> currently is no support to merge the snapshot changes back into the
> original image while the VM is running, but I think that is the next
> planned step.

Yes, keep in mind that the live snapshot is only for external snapshot
files, it doesn't deal with internal snapshots.

Cheers,
Jes