Re: [PATCH 1/2] copy-before-write: allow specifying minimum cluster size

[Markus Armbruster <armbru@redhat.com>]

Fiona Ebner <f.ebner@proxmox.com> writes:

> Useful to make discard-source work in the context of backup fleecing
> when the fleecing image has a larger granularity than the backup
> target.
>
> Copy-before-write operations will use at least this granularity and in
> particular, discard requests to the source node will too. If the
> granularity is too small, they will just be aligned down in
> cbw_co_pdiscard_snapshot() and thus effectively ignored.
>
> The QAPI uses uint32 so the value will be non-negative, but still fit
> into a uint64_t.
>
> Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
>  block/block-copy.c         | 17 +++++++++++++----
>  block/copy-before-write.c  |  3 ++-
>  include/block/block-copy.h |  1 +
>  qapi/block-core.json       |  8 +++++++-
>  4 files changed, 23 insertions(+), 6 deletions(-)
>
> diff --git a/block/block-copy.c b/block/block-copy.c
> index 7e3b378528..adb1cbb440 100644
> --- a/block/block-copy.c
> +++ b/block/block-copy.c
> @@ -310,6 +310,7 @@ void block_copy_set_copy_opts(BlockCopyState *s, bool use_copy_range,
>  }
>  
>  static int64_t block_copy_calculate_cluster_size(BlockDriverState *target,
> +                                                 int64_t min_cluster_size,
>                                                   Error **errp)
>  {
>      int ret;
> @@ -335,7 +336,7 @@ static int64_t block_copy_calculate_cluster_size(BlockDriverState *target,
>                      "used. If the actual block size of the target exceeds "
>                      "this default, the backup may be unusable",
>                      BLOCK_COPY_CLUSTER_SIZE_DEFAULT);
> -        return BLOCK_COPY_CLUSTER_SIZE_DEFAULT;
> +        return MAX(min_cluster_size, BLOCK_COPY_CLUSTER_SIZE_DEFAULT);

min_cluster_size is int64_t, BLOCK_COPY_CLUSTER_SIZE_DEFAULT is int, and
gets converted to int64_t.  The return type is int64_t.  Okay.

>      } else if (ret < 0 && !target_does_cow) {
>          error_setg_errno(errp, -ret,
>              "Couldn't determine the cluster size of the target image, "
> @@ -345,16 +346,18 @@ static int64_t block_copy_calculate_cluster_size(BlockDriverState *target,
>          return ret;
>      } else if (ret < 0 && target_does_cow) {
>          /* Not fatal; just trudge on ahead. */
> -        return BLOCK_COPY_CLUSTER_SIZE_DEFAULT;
> +        return MAX(min_cluster_size, BLOCK_COPY_CLUSTER_SIZE_DEFAULT);

Same.

>      }
>  
> -    return MAX(BLOCK_COPY_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
> +    return MAX(min_cluster_size,
> +               MAX(BLOCK_COPY_CLUSTER_SIZE_DEFAULT, bdi.cluster_size));

Similar: bdi.cluster_size is int.

>  }
>  
>  BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
>                                       BlockDriverState *copy_bitmap_bs,
>                                       const BdrvDirtyBitmap *bitmap,
>                                       bool discard_source,
> +                                     int64_t min_cluster_size,
>                                       Error **errp)
>  {
>      ERRP_GUARD();
> @@ -365,7 +368,13 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
>  
>      GLOBAL_STATE_CODE();
>  
> -    cluster_size = block_copy_calculate_cluster_size(target->bs, errp);
> +    if (min_cluster_size && !is_power_of_2(min_cluster_size)) {

min_cluster_size is int64_t, is_power_of_2() takes uint64_t.  Bad if
min_cluster_size is negative.  Could this happen?


> +        error_setg(errp, "min-cluster-size needs to be a power of 2");
> +        return NULL;
> +    }
> +
> +    cluster_size = block_copy_calculate_cluster_size(target->bs,
> +                                                     min_cluster_size, errp);

min_cluster_size is int64_t, block_copy_calculate_cluster_size() takes
int64_t, returns int64_t, and cluster_size is int64_t.  Good.

>      if (cluster_size < 0) {
>          return NULL;
>      }
> diff --git a/block/copy-before-write.c b/block/copy-before-write.c
> index dac57481c5..f9896c6c1e 100644
> --- a/block/copy-before-write.c
> +++ b/block/copy-before-write.c
> @@ -476,7 +476,8 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
>  
>      s->discard_source = flags & BDRV_O_CBW_DISCARD_SOURCE;
>      s->bcs = block_copy_state_new(bs->file, s->target, bs, bitmap,
> -                                  flags & BDRV_O_CBW_DISCARD_SOURCE, errp);
> +                                  flags & BDRV_O_CBW_DISCARD_SOURCE,
> +                                  opts->min_cluster_size, errp);

@opts is BlockdevOptionsCbw *, opts->min_cluster_size is uint32_t (see
last hunk), block_copy_state_new() takes int64_t: opts->min_cluster_size
gets zero-extended.  Okay.

>      if (!s->bcs) {
>          error_prepend(errp, "Cannot create block-copy-state: ");
>          return -EINVAL;
> diff --git a/include/block/block-copy.h b/include/block/block-copy.h
> index bdc703bacd..77857c6c68 100644
> --- a/include/block/block-copy.h
> +++ b/include/block/block-copy.h
> @@ -28,6 +28,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
>                                       BlockDriverState *copy_bitmap_bs,
>                                       const BdrvDirtyBitmap *bitmap,
>                                       bool discard_source,
> +                                     int64_t min_cluster_size,
>                                       Error **errp);
>  
>  /* Function should be called prior any actual copy request */
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 0a72c590a8..85c8f88f6e 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -4625,12 +4625,18 @@
>  #     @on-cbw-error parameter will decide how this failure is handled.
>  #     Default 0. (Since 7.1)
>  #
> +# @min-cluster-size: Minimum size of blocks used by copy-before-write
> +#     operations.  Has to be a power of 2.  No effect if smaller than
> +#     the maximum of the target's cluster size and 64 KiB.  Default 0.
> +#     (Since 9.0)
> +#
>  # Since: 6.2
>  ##
>  { 'struct': 'BlockdevOptionsCbw',
>    'base': 'BlockdevOptionsGenericFormat',
>    'data': { 'target': 'BlockdevRef', '*bitmap': 'BlockDirtyBitmap',
> -            '*on-cbw-error': 'OnCbwError', '*cbw-timeout': 'uint32' } }
> +            '*on-cbw-error': 'OnCbwError', '*cbw-timeout': 'uint32',
> +            '*min-cluster-size': 'uint32' } }

Elsewhere in the schema, we use either 'int' or 'size' for cluster-size.
Why the difference?

>  
>  ##
>  # @BlockdevOptions: