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qemu-img: add compression option to rebase subcommand

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If we rebase an image whose backing file has compressed clusters, we
might end up wasting disk space since the copied clusters are now
uncompressed.  In order to have better control over this, let's add
"--compress" option to the "qemu-img rebase" command.

Note that this option affects only the clusters which are actually being
copied from the original backing file.  The clusters which were
uncompressed in the target image will remain so.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20230919165804.439110-8-andrey.drobyshev@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Andrey Drobyshev 2023-09-19 19:58:03 +03:00 • committed by Kevin Wolf
parent f93e65ee51
commit 26ea27898b
3 changed files with 26 additions and 10 deletions
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6
docs/tools/qemu-img.rst
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@ -667,7 +667,7 @@ Command description:
List, apply, create or delete snapshots in image *FILENAME*.
.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME
.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] [-c] -b BACKING_FILE [-F BACKING_FMT] FILENAME
Changes the backing file of an image. Only the formats ``qcow2`` and
``qed`` support changing the backing file.
@ -694,7 +694,9 @@ Command description:
In order to achieve this, any clusters that differ between
*BACKING_FILE* and the old backing file of *FILENAME* are merged
into *FILENAME* before actually changing the backing file.
into *FILENAME* before actually changing the backing file. With the
``-c`` option specified, the clusters which are being merged (but not
the entire *FILENAME* image) are compressed when written.
Note that the safe mode is an expensive operation, comparable to
converting an image. It only works if the old backing file still