Merge remote-tracking branch 'kwolf/for-anthony' into staging

* kwolf/for-anthony:
  fdc-test: introduced qtest no_media_on_start and cmos qtest for floppy
  fdc: fix media detection
  fdc: floppy drive should be visible after start without media
  qemu-iotests: mark 035 qcow2-only
  qcow2: Check qcow2_alloc_clusters_at() return value
  sheepdog: use heap instead of stack for BDRVSheepdogState
  sheepdog: return -errno on error
  sheepdog: mark image as snapshot when tag is specified
  qemu-img: Explain how rebase operation can be used to perform a 'diff' operation.
  qcow2: don't leak buffer for unexpected qcow_version in header

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:
+
+@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}.