Re: [PATCH 1/3] block: add subcluster_size field to BlockDriverInfo

["Denis V. Lunev" <den@virtuozzo.com>]

On 6/26/23 18:08, Andrey Drobyshev wrote:
> This is going to be used in the subsequent commit as requests alignment
> (in particular, during copy-on-read).  This value only makes sense for
> the formats which support subclusters (currently QCOW2 only).  If this
> field isn't set by driver's own bdrv_get_info() implementation, we
> simply set it equal to the cluster size thus treating each cluster as having
> a single subcluster.
>
> Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> ---
>   block.c                      | 7 +++++++
>   block/qcow2.c                | 1 +
>   include/block/block-common.h | 5 +++++
>   3 files changed, 13 insertions(+)
>
> diff --git a/block.c b/block.c
> index 0637265c26..4fe1743cfb 100644
> --- a/block.c
> +++ b/block.c
> @@ -6392,6 +6392,13 @@ int coroutine_fn bdrv_co_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
>       }
>       memset(bdi, 0, sizeof(*bdi));
>       ret = drv->bdrv_co_get_info(bs, bdi);
> +    if (bdi->subcluster_size == 0) {
> +        /*
> +         * If the driver left this unset, subclusters either not supported.
> +         * Then it is safe to treat each cluster as having only one subcluster.
> +         */
> +        bdi->subcluster_size = bdi->cluster_size;
> +    }
>       if (ret < 0) {
>           return ret;
>       }
> diff --git a/block/qcow2.c b/block/qcow2.c
> index e23edd48c2..94b8d0e1c2 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -5197,6 +5197,7 @@ qcow2_co_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
>   {
>       BDRVQcow2State *s = bs->opaque;
>       bdi->cluster_size = s->cluster_size;
> +    bdi->subcluster_size = s->subcluster_size;
>       bdi->vm_state_offset = qcow2_vm_state_offset(s);
>       bdi->is_dirty = s->incompatible_features & QCOW2_INCOMPAT_DIRTY;
>       return 0;
> diff --git a/include/block/block-common.h b/include/block/block-common.h
> index e15395f2cb..df5ffc8d09 100644
> --- a/include/block/block-common.h
> +++ b/include/block/block-common.h
> @@ -132,6 +132,11 @@ typedef struct BlockZoneWps {
>   typedef struct BlockDriverInfo {
>       /* in bytes, 0 if irrelevant */
>       int cluster_size;
> +    /*
> +     * A fraction of cluster_size, if supported (currently QCOW2 only); if
> +     * disabled or unsupported, set equal to cluster_size.
> +     */
> +    int subcluster_size;
>       /* offset at which the VM state can be saved (0 if not possible) */
>       int64_t vm_state_offset;
>       bool is_dirty;
Reviewed-by: Denis V. Lunev <den@openvz.org>