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Re: [Qemu-devel] [PATCH v3] qemu-img: Implement 'diff' operation.

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] [PATCH v3] qemu-img: Implement 'diff' operation.
Date: Mon, 21 May 2012 14:59:26 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, May 21, 2012 at 03:29:22PM +0200, Kevin Wolf wrote:
> Am 17.05.2012 17:34, schrieb Richard W.M. Jones:
> > From: "Richard W.M. Jones" <address@hidden>
> > 
> > This produces a qcow2 file which is the difference between
> > two disk images.  ie, if:
> > 
> >   base.img     - is a disk image (in any format)
> >   modified.img - is base.img, copied and modified
> > 
> > then:
> > 
> >   qemu-img diff -b base.img modified.img diff.qcow2
> > 
> > creates 'diff.qcow2' which contains the differences between 'base.img'
> > and 'modified.img'.  Note that 'diff.qcow2' has 'base.img' as its
> > backing file.
> > 
> > Signed-off-by: Richard W.M. Jones <address@hidden>
> > Cc: Matthew Booth <address@hidden>
> > Cc: Pablo Iranzo Gómez <address@hidden>
> > Cc: Tomas Von Veschler <address@hidden>
> Hm, I'm wondering... If I have a command line like this:
>   qemu-img diff -b base.img modified.img diff.qcow2
> Would this be equivalent to this sequence?
>   qemu-img create -f qcow2 -b modified.img diff.qcow2
>   qemu-img rebase -b base.img diff.qcow2
> Or is there some detail that I'm missing? If it is equivalent, this
> would suggest that either the new command isn't necessary at all or at
> least that it should reuse the qemu-img rebase code.

Yes.  I tried for a while to work out the sequence of commands that
could make a diff using 'qemu-img rebase', but it wasn't obvious and I
gave up.  It should at least be documented.  How about the attached


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming blog: http://rwmj.wordpress.com
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Attachment: 0001-qemu-img-Explain-how-rebase-operation-can-be-used-to.patch
Description: Text document

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