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

[Eric Blake]

On 05/17/2012 07:52 AM, Peter Maydell wrote:
> 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...

Even something as simple as:

Useful for converting a monolithic image back into a thin image on top
of a common base.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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