Re: [Qemu-devel] [PATCH] qemu-img: Implement 'diff' operation.

[Peter Maydell]

On 17 May 2012 14:44, Richard W.M. Jones <address@hidden> wrote:
> From: "Richard W.M. Jones" <address@hidden>
>
> This produces a qcow2 file which is the different between
> two disk images.  ie, if:
>
>  original.img - is a disk image (in any format)
>  modified.img - is a modified version of original.img
>
> then:
>
>  qemu-img diff -b original.img modified.img diff.qcow2
>
> creates 'diff.qcow2' which contains just the differences.  Note that
> 'diff.qcow2' has 'original.img' set as the backing file.

Any chance of some more detailed explanation in the docs patch
about what this actually means and why it's useful? I spent
several minutes going "huh, does it even mean anything to
calculate the difference between two binary disk images?"
before realising that it's the presence of the backing file
that makes it actually make sense...

(maybe I'm just dense :-))

-- PMM