[Qemu-devel] Derivative snapshots or snapshots for versioning

[Qemu-devel] Derivative snapshots or snapshots for versioning

[Mike Young]

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Hi,

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

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?

Thanks,

Mike



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

[Stefan Hajnoczi]

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

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

[Mike Young]

Thanks Stefan!

On 5/25/11 1:21 AM, "Stefan Hajnoczi" <stefanha@gmail.com> wrote:

>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