Re: [Qemu-devel] [PATCH 0/2] Update documentation for qemu-img + add new -B option

["Marc Bevand" <m.bevand@gmail.com>]

On Wed, Jun 4, 2008 at 12:37 AM, Jamie Lokier <jamie@shareable.org> wrote:
>
> I wonder if a useful option would be to be able to create an image
> relative to a _different_ base image.  I.e.

I see what you mean. qemu-img could compare the content of each sector
in images A and B (no matter what A is based on) and infer and image
A' (based on B) equivalent to A.

I wonder too if it would be _that_ useful in real-world scenarios.

> Another thing, which might be easier and more useful, would be
> instantiating zeros in the source image into holes in the relative
> image.  To save space, after lots of changes on the virtual hard disk
> and some zeroing.  (Microsoft has a tool to do this with Virtual PC).

Actually "qemu-img convert A B" already does this: zeros in A are
converted to holes in B (more precisely: real holes if B is a raw
image, or undefined clusters if the qcow/qcow2 case). B in this
example would be a flat image.

If you wanted to do the same thing with B a relative image, the
qcow/qcow2 format would need to be extended because there is currently
no defined way to efficiently represent zero'd clusters in relative
images. (Or actually now that I think about it maybe we could
represent zero'd clusters as if they were compressed even when
compression is disabled.)

I am curious, does Virtual PC support relative aka copy-on-write images ?

-- 
Marc Bevand