From: Eric Blake <eblake@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Richard W.M. Jones" <rjones@redhat.com>, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH] qemu-img: Implement 'diff' operation.
Date: Thu, 17 May 2012 07:58:53 -0600 [thread overview]
Message-ID: <4FB5041D.3050402@redhat.com> (raw)
In-Reply-To: <CAFEAcA_KC9nJht9Q7ZmZ-E_wYapdNNE-X+7JXEbDgxvzzgSE5A@mail.gmail.com>
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On 05/17/2012 07:52 AM, Peter Maydell wrote:
> On 17 May 2012 14:44, Richard W.M. Jones <rjones@redhat.com> wrote:
>> From: "Richard W.M. Jones" <rjones@redhat.com>
>>
>> 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 eblake@redhat.com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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next prev parent reply other threads:[~2012-05-17 13:59 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-17 13:44 [Qemu-devel] [PATCH] qemu-img: Implement 'diff' operation Richard W.M. Jones
2012-05-17 13:52 ` Peter Maydell
2012-05-17 13:58 ` Eric Blake [this message]
2012-05-17 14:01 ` Richard W.M. Jones
2012-05-17 13:57 ` Eric Blake
2012-05-17 14:58 ` Richard W.M. Jones
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