block: update block commit documentation regarding image truncation
This updates the documentation for commiting snapshot images.
Specifically, this highlights what happens when the base image
is either smaller or larger than the snapshot image being committed.
In the case of the base image being smaller, it is resized to the
larger size of the snapshot image. In the case of the base image
being larger, it is not resized automatically, but once the commit
has completed it is safe for the user to truncate the base image.
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index feca084..f3fc514 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -35,6 +35,11 @@
@item commit
@findex commit
Commit changes to the disk images (if -snapshot is used) or backing files.
+If the backing file is smaller than the snapshot, then the backing file will be
+resized to be the same size as the snapshot. If the snapshot is smaller than
+the backing file, the backing file will not be truncated. If you want the
+backing file to match the size of the smaller snapshot, you can safely truncate
+it yourself once the commit operation successfully completes.
ETEXI
{
