path: root/qemu-img.texi
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authorRichard W.M. Jones <rjones@redhat.com>2012-05-21 14:58:05 +0100
committerKevin Wolf <kwolf@redhat.com>2012-05-25 18:12:54 +0200
commit9fda6ab1d9f33c04f35438bce101427dd557fef6 (patch)
treea926c774e3beb7f42911d18d91818f429c0fc079 /qemu-img.texi
parentb6c147622d31272f9728da9ec16d146bf8c45a74 (diff)
qemu-img: Explain how rebase operation can be used to perform a 'diff' operation.
Signed-off-by: Richard W.M. Jones <rjones@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'qemu-img.texi')
1 files changed, 18 insertions, 0 deletions
diff --git a/qemu-img.texi b/qemu-img.texi
index b2ca3a542c..6fc3c28e0d 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:
+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}.