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block: update block commit documentation regarding image truncation

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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>
This commit is contained in:
Jeff Cody 2014-01-24 09:02:37 -05:00 • committed by Kevin Wolf
parent 4da8358596
commit 3722290074
4 changed files with 57 additions and 1 deletions
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7
qemu-img.texi
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@ -142,7 +142,12 @@ it doesn't need to be specified separately in this case.
@item commit [-f @var{fmt}] [-t @var{cache}] @var{filename}
Commit the changes recorded in @var{filename} in its base image.
Commit the changes recorded in @var{filename} in its base image or backing file.
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.
@item compare [-f @var{fmt}] [-F @var{fmt}] [-p] [-s] [-q] @var{filename1} @var{filename2}