Re: [PATCH 5/6] qemu-img: add compression option to rebase subcommand

[Hanna Czenczek <hreitz@redhat.com>]

On 01.06.23 21:28, Andrey Drobyshev via wrote:
> 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>
> ---
>   docs/tools/qemu-img.rst |  6 ++++--
>   qemu-img-cmds.hx        |  4 ++--
>   qemu-img.c              | 19 +++++++++++++++++--
>   3 files changed, 23 insertions(+), 6 deletions(-)

Interesting.  I was about to protest because we only really support 
writing compressed clusters to new and empty images, so the qcow2 driver 
does not allow overwriting existing clusters with compressed data.  But 
by design we skip all clusters that are anything but unallocated in the 
top image (i.e. the one we are going to write to), so this should indeed 
work out well.

Reviewed-by: Hanna Czenczek <hreitz@redhat.com>

> diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
> index 15aeddc6d8..973a912dec 100644
> --- a/docs/tools/qemu-img.rst
> +++ b/docs/tools/qemu-img.rst
> @@ -663,7 +663,7 @@ 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.
> @@ -690,7 +690,9 @@ 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 ``-c``

“With the ``-c`` option specified, [...]”

> +    option specified, the clusters which are being merged (but not the
> +    entire *FILENAME* image) are written in the compressed mode.

“[...] 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