Re: [Qemu-devel] Derivative snapshots or snapshots for versioning

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, May 24, 2011 at 5:14 PM, Mike Young <myoung@wildernessvoice.com> wrote:
> I'm trying to see if I can I do a derivative snapshot as a means of
> versioning. I wish to do this vs dd or cp as it's much faster. I do not
> intend to apply a snapshot back to an original volume.
> So, let's say I have original_volume.img and I create a snapshot using the
> –b option: "qemu-img create –f qcow2 –b original_volume.img snapshot1.img",

I would not call the new file "snapshot1.img" since the frozen disk
image should be original_volume.img.  You can write to snapshot1.img
but it is not safe to original_volume.img because snapshot1.img will
then become inconsistent (you've changed the blocks that it was based
on).

> but now I wish to create a snapshot2 from snapshot1.img. Is there a way to
> do this?

qemu-img create -f qcow2 -b snapshot1.img snapshot2.img

Note that you should not write to snapshot1.img anymore.  However, you
could say:

qemu-img create -f qcow2 -b snapshot1.img snapshot2a.img
qemu-img create -f qcow2 -b snapshot1.img snapshot2b.img

Don't write to snapshot1.img but you can run VMs using snapshot2a.img
and snapshot2b.img independently.

> Also, is there a way to generate 100% snapshot of the original volume (an
> exact replica), so that a 40G original results in a 40G snapshot?

cp original.qcow2 clone.qcow2

Stefan