Re: [PATCH for-5.1 1/2] qcow2: Implement v2 zero writes with discard if possible

[Nir Soffer]

On Mon, Jul 20, 2020 at 4:18 PM Kevin Wolf <kwolf@redhat.com> wrote:
>
> qcow2 version 2 images don't support the zero flag for clusters, so for
> write_zeroes requests, we return -ENOTSUP and get explicit zero buffer
> writes. If the image doesn't have a backing file, we can do better: Just
> discard the respective clusters.
>
> This is relevant for 'qemu-img convert -O qcow2 -n', where qemu-img has
> to assume that the existing target image may contain any data, so it has
> to write zeroes. Without this patch, this results in a fully allocated
> target image, even if the source image was empty.
>
> Reported-by: Nir Soffer <nsoffer@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/qcow2-cluster.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 4b5fc8c4a7..a677ba9f5c 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -1797,8 +1797,15 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, 
> uint64_t offset,
>      assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
>             end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
>
> -    /* The zero flag is only supported by version 3 and newer */
> +    /*
> +     * The zero flag is only supported by version 3 and newer. However, if we
> +     * have no backing file, we can resort to discard in version 2.
> +     */
>      if (s->qcow_version < 3) {
> +        if (!bs->backing) {
> +            return qcow2_cluster_discard(bs, offset, bytes,
> +                                         QCOW2_DISCARD_REQUEST, false);
> +        }
>          return -ENOTSUP;
>      }

Looks good to me.

>
> --
> 2.25.4
>