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" <rjo...@redhat.com>
> >
> > 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 <rjo...@redhat.com>
> > Cc: Matthew Booth <mbo...@redhat.com>
> > Cc: Pablo Iranzo Gómez <pablo.ira...@redhat.com>
> > Cc: Tomas Von Veschler <tvv...@redhat.com>
>
> 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
patch?
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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>From eec7fa864298b5619c52ada1e8fbd32c00b302d9 Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rjo...@redhat.com>
Date: Mon, 21 May 2012 14:58:05 +0100
Subject: [PATCH] qemu-img: Explain how rebase operation can be used to
perform a 'diff' operation.
Signed-off-by: Richard W.M. Jones <rjo...@redhat.com>
---
qemu-img.texi | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)
diff --git a/qemu-img.texi b/qemu-img.texi
index b2ca3a5..6fc3c28 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -159,6 +159,24 @@ It can be used without an accessible old backing file,
i.e. you can use it to
fix an image whose backing file has already been moved/renamed.
@end table
+You can use @code{rebase} to perform a ``diff'' operation on two
+disk images. This can be useful when you have copied or cloned
+a guest, and you want to get back to a thin image on top of a
+template or base image.
+
+Say that @code{base.img} has been cloned as @code{modified.img} by
+copying it, and that the @code{modified.img} guest has run so there
+are now some changes compared to @code{base.img}. To construct a thin
+image called @code{diff.qcow2} that contains just the differences, do:
+
+@example
+qemu-img create -f qcow2 -b modified.img diff.qcow2
+qemu-img rebase -b base.img diff.qcow2
+@end example
+
+At this point, @code{modified.img} can be discarded, since
+@code{base.img + diff.qcow2} contains the same information.
+
@item resize @var{filename} [+ | -]@var{size}
Change the disk image as if it had been created with @var{size}.
--
1.7.10
