In the context of backup fleecing, discarding the source will not work
when the fleecing image has a larger granularity than the one used for
block-copy operations (can happen if the backup target has smaller
cluster size), because cbw_co_pdiscard_snapshot() will align down the
discard requests and thus effectively ignore then.

To make @discard-source work in such a scenario, allow specifying the
minimum cluster size used for block-copy operations and thus in
particular also the granularity for discard requests to the source.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Acked-by: Markus Armbruster <armbru@redhat.com> (QAPI schema)
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Changes in v3:
* Use PRI{i,u}64 macros
* Do not leak qdict in error case

 block/backup.c            | 2 +-
 block/copy-before-write.c | 9 +++++++++
 block/copy-before-write.h | 1 +
 blockdev.c                | 3 +++
 qapi/block-core.json      | 9 +++++++--
 5 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/block/backup.c b/block/backup.c
index 3dd2e229d2..a1292c01ec 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -458,7 +458,7 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
     }
 
     cbw = bdrv_cbw_append(bs, target, filter_node_name, discard_source,
-                          &bcs, errp);
+                          perf->min_cluster_size, &bcs, errp);
     if (!cbw) {
         goto error;
     }
diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index a919b1f41b..e835987e52 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -548,6 +548,7 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockDriverState *target,
                                   const char *filter_node_name,
                                   bool discard_source,
+                                  uint64_t min_cluster_size,
                                   BlockCopyState **bcs,
                                   Error **errp)
 {
@@ -567,6 +568,14 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
     qdict_put_str(opts, "file", bdrv_get_node_name(source));
     qdict_put_str(opts, "target", bdrv_get_node_name(target));
 
+    if (min_cluster_size > INT64_MAX) {
+        error_setg(errp, "min-cluster-size too large: %" PRIu64 " > %" PRIi64,
+                   min_cluster_size, INT64_MAX);
+        qobject_unref(opts);
+        return NULL;
+    }
+    qdict_put_int(opts, "min-cluster-size", (int64_t)min_cluster_size);
+
     top = bdrv_insert_node(source, opts, flags, errp);
     if (!top) {
         return NULL;
diff --git a/block/copy-before-write.h b/block/copy-before-write.h
index 01af0cd3c4..2a5d4ba693 100644
--- a/block/copy-before-write.h
+++ b/block/copy-before-write.h
@@ -40,6 +40,7 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockDriverState *target,
                                   const char *filter_node_name,
                                   bool discard_source,
+                                  uint64_t min_cluster_size,
                                   BlockCopyState **bcs,
                                   Error **errp);
 void bdrv_cbw_drop(BlockDriverState *bs);
diff --git a/blockdev.c b/blockdev.c
index 835064ed03..6740663fda 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2655,6 +2655,9 @@ static BlockJob *do_backup_common(BackupCommon *backup,
         if (backup->x_perf->has_max_chunk) {
             perf.max_chunk = backup->x_perf->max_chunk;
         }
+        if (backup->x_perf->has_min_cluster_size) {
+            perf.min_cluster_size = backup->x_perf->min_cluster_size;
+        }
     }
 
     if ((backup->sync == MIRROR_SYNC_MODE_BITMAP) ||
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 80e32db8aa..9a54bfb15f 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1551,11 +1551,16 @@
 #     it should not be less than job cluster size which is calculated
 #     as maximum of target image cluster size and 64k.  Default 0.
 #
+# @min-cluster-size: Minimum size of blocks used by copy-before-write
+#     and background copy 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 9.1)
+#
 # Since: 6.0
 ##
 { 'struct': 'BackupPerf',
-  'data': { '*use-copy-range': 'bool',
-            '*max-workers': 'int', '*max-chunk': 'int64' } }
+  'data': { '*use-copy-range': 'bool', '*max-workers': 'int',
+            '*max-chunk': 'int64', '*min-cluster-size': 'size' } }
 
 ##
 # @BackupCommon:
-- 
2.39.2

On 11.07.24 15:09, Fiona Ebner wrote:
> In the context of backup fleecing, discarding the source will not work
> when the fleecing image has a larger granularity than the one used for
> block-copy operations (can happen if the backup target has smaller
> cluster size), because cbw_co_pdiscard_snapshot() will align down the
> discard requests and thus effectively ignore then.
> 
> To make @discard-source work in such a scenario, allow specifying the
> minimum cluster size used for block-copy operations and thus in
> particular also the granularity for discard requests to the source.
> 
> Suggested-by: Vladimir Sementsov-Ogievskiy<vsementsov@yandex-team.ru>
> Acked-by: Markus Armbruster<armbru@redhat.com>  (QAPI schema)
> Signed-off-by: Fiona Ebner<f.ebner@proxmox.com>

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

-- 
Best regards,
Vladimir