diff --git a/block/copy-before-write.h b/block/copy-before-write.h
index 538aab8bdb..b386fd8f01 100644
--- a/block/copy-before-write.h
+++ b/block/copy-before-write.h
@@ -32,7 +32,6 @@
 BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockDriverState *target,
                                   const char *filter_node_name,
-                                  uint64_t cluster_size,
                                   bool compress,
                                   BlockCopyState **bcs,
                                   Error **errp);
diff --git a/include/block/block-copy.h b/include/block/block-copy.h
index b74fb80067..e81cd952f7 100644
--- a/include/block/block-copy.h
+++ b/include/block/block-copy.h
@@ -23,8 +23,8 @@ typedef struct BlockCopyState BlockCopyState;
 typedef struct BlockCopyCallState BlockCopyCallState;
 
 BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
-                                     int64_t cluster_size, bool use_copy_range,
-                                     bool compress, Error **errp);
+                                     bool use_copy_range, bool compress,
+                                     Error **errp);
 
 void block_copy_set_copy_opts(BlockCopyState *s, bool use_copy_range,
                               bool compress);
@@ -88,6 +88,7 @@ void block_copy_kick(BlockCopyCallState *call_state);
 void block_copy_call_cancel(BlockCopyCallState *call_state);
 
 BdrvDirtyBitmap *block_copy_dirty_bitmap(BlockCopyState *s);
+int64_t block_copy_cluster_size(BlockCopyState *s);
 void block_copy_set_skip_unallocated(BlockCopyState *s, bool skip);
 
 #endif /* BLOCK_COPY_H */
diff --git a/block/backup.c b/block/backup.c
index b31fd99ab3..83516297cb 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -29,8 +29,6 @@
 
 #include "block/copy-before-write.h"
 
-#define BACKUP_CLUSTER_SIZE_DEFAULT (1 << 16)
-
 typedef struct BackupBlockJob {
     BlockJob common;
     BlockDriverState *cbw;
@@ -354,43 +352,6 @@ static const BlockJobDriver backup_job_driver = {
     .set_speed = backup_set_speed,
 };
 
-static int64_t backup_calculate_cluster_size(BlockDriverState *target,
-                                             Error **errp)
-{
-    int ret;
-    BlockDriverInfo bdi;
-    bool target_does_cow = bdrv_backing_chain_next(target);
-
-    /*
-     * If there is no backing file on the target, we cannot rely on COW if our
-     * backup cluster size is smaller than the target cluster size. Even for
-     * targets with a backing file, try to avoid COW if possible.
-     */
-    ret = bdrv_get_info(target, &bdi);
-    if (ret == -ENOTSUP && !target_does_cow) {
-        /* Cluster size is not defined */
-        warn_report("The target block device doesn't provide "
-                    "information about the block size and it doesn't have a "
-                    "backing file. The default block size of %u bytes is "
-                    "used. If the actual block size of the target exceeds "
-                    "this default, the backup may be unusable",
-                    BACKUP_CLUSTER_SIZE_DEFAULT);
-        return BACKUP_CLUSTER_SIZE_DEFAULT;
-    } else if (ret < 0 && !target_does_cow) {
-        error_setg_errno(errp, -ret,
-            "Couldn't determine the cluster size of the target image, "
-            "which has no backing file");
-        error_append_hint(errp,
-            "Aborting, since this may create an unusable destination image\n");
-        return ret;
-    } else if (ret < 0 && target_does_cow) {
-        /* Not fatal; just trudge on ahead. */
-        return BACKUP_CLUSTER_SIZE_DEFAULT;
-    }
-
-    return MAX(BACKUP_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
-}
-
 BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
@@ -448,11 +409,6 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         return NULL;
     }
 
-    cluster_size = backup_calculate_cluster_size(target, errp);
-    if (cluster_size < 0) {
-        goto error;
-    }
-
     if (perf->max_workers < 1) {
         error_setg(errp, "max-workers must be greater than zero");
         return NULL;
@@ -464,13 +420,6 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         return NULL;
     }
 
-    if (perf->max_chunk && perf->max_chunk < cluster_size) {
-        error_setg(errp, "Required max-chunk (%" PRIi64 ") is less than backup "
-                   "cluster size (%" PRIi64 ")", perf->max_chunk, cluster_size);
-        return NULL;
-    }
-
-
     if (sync_bitmap) {
         /* If we need to write to this bitmap, check that we can: */
         if (bitmap_mode != BITMAP_SYNC_MODE_NEVER &&
@@ -503,12 +452,19 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
         goto error;
     }
 
-    cbw = bdrv_cbw_append(bs, target, filter_node_name,
-                          cluster_size, false, &bcs, errp);
+    cbw = bdrv_cbw_append(bs, target, filter_node_name, false, &bcs, errp);
     if (!cbw) {
         goto error;
     }
 
+    cluster_size = block_copy_cluster_size(bcs);
+
+    if (perf->max_chunk && perf->max_chunk < cluster_size) {
+        error_setg(errp, "Required max-chunk (%" PRIi64 ") is less than backup "
+                   "cluster size (%" PRIi64 ")", perf->max_chunk, cluster_size);
+        goto error;
+    }
+
     /* job->len is fixed, so we can't allow resize */
     job = block_job_create(job_id, &backup_job_driver, txn, cbw,
                            0, BLK_PERM_ALL,
diff --git a/block/block-copy.c b/block/block-copy.c
index 8baf8b9403..774f5029bb 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -27,6 +27,7 @@
 #define BLOCK_COPY_MAX_MEM (128 * MiB)
 #define BLOCK_COPY_MAX_WORKERS 64
 #define BLOCK_COPY_SLICE_TIME 100000000ULL /* ns */
+#define BLOCK_COPY_CLUSTER_SIZE_DEFAULT (1 << 16)
 
 static coroutine_fn int block_copy_task_entry(AioTask *task);
 
@@ -276,13 +277,56 @@ void block_copy_set_copy_opts(BlockCopyState *s, bool use_copy_range,
     }
 }
 
+static int64_t block_copy_calculate_cluster_size(BlockDriverState *target,
+                                                 Error **errp)
+{
+    int ret;
+    BlockDriverInfo bdi;
+    bool target_does_cow = bdrv_backing_chain_next(target);
+
+    /*
+     * If there is no backing file on the target, we cannot rely on COW if our
+     * backup cluster size is smaller than the target cluster size. Even for
+     * targets with a backing file, try to avoid COW if possible.
+     */
+    ret = bdrv_get_info(target, &bdi);
+    if (ret == -ENOTSUP && !target_does_cow) {
+        /* Cluster size is not defined */
+        warn_report("The target block device doesn't provide "
+                    "information about the block size and it doesn't have a "
+                    "backing file. The default block size of %u bytes is "
+                    "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;
+    } else if (ret < 0 && !target_does_cow) {
+        error_setg_errno(errp, -ret,
+            "Couldn't determine the cluster size of the target image, "
+            "which has no backing file");
+        error_append_hint(errp,
+            "Aborting, since this may create an unusable destination image\n");
+        return ret;
+    } else if (ret < 0 && target_does_cow) {
+        /* Not fatal; just trudge on ahead. */
+        return BLOCK_COPY_CLUSTER_SIZE_DEFAULT;
+    }
+
+    return MAX(BLOCK_COPY_CLUSTER_SIZE_DEFAULT, bdi.cluster_size);
+}
+
 BlockCopyState *block_copy_state_new(BdrvChild *source, BdrvChild *target,
-                                     int64_t cluster_size, bool use_copy_range,
+                                     bool use_copy_range,
                                      bool compress, Error **errp)
 {
     BlockCopyState *s;
+    int64_t cluster_size;
     BdrvDirtyBitmap *copy_bitmap;
 
+    cluster_size = block_copy_calculate_cluster_size(target->bs, errp);
+    if (cluster_size < 0) {
+        return NULL;
+    }
+
     copy_bitmap = bdrv_create_dirty_bitmap(source->bs, cluster_size, NULL,
                                            errp);
     if (!copy_bitmap) {
@@ -879,6 +923,11 @@ BdrvDirtyBitmap *block_copy_dirty_bitmap(BlockCopyState *s)
     return s->copy_bitmap;
 }
 
+int64_t block_copy_cluster_size(BlockCopyState *s)
+{
+    return s->cluster_size;
+}
+
 void block_copy_set_skip_unallocated(BlockCopyState *s, bool skip)
 {
     s->skip_unallocated = skip;
diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 235251a640..a7996d54db 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -37,7 +37,6 @@
 typedef struct BDRVCopyBeforeWriteState {
     BlockCopyState *bcs;
     BdrvChild *target;
-    int64_t cluster_size;
 } BDRVCopyBeforeWriteState;
 
 static coroutine_fn int cbw_co_preadv(
@@ -52,13 +51,14 @@ static coroutine_fn int cbw_do_copy_before_write(BlockDriverState *bs,
 {
     BDRVCopyBeforeWriteState *s = bs->opaque;
     uint64_t off, end;
+    int64_t cluster_size = block_copy_cluster_size(s->bcs);
 
     if (flags & BDRV_REQ_WRITE_UNCHANGED) {
         return 0;
     }
 
-    off = QEMU_ALIGN_DOWN(offset, s->cluster_size);
-    end = QEMU_ALIGN_UP(offset + bytes, s->cluster_size);
+    off = QEMU_ALIGN_DOWN(offset, cluster_size);
+    end = QEMU_ALIGN_UP(offset + bytes, cluster_size);
 
     return block_copy(s->bcs, off, end - off, true);
 }
@@ -169,7 +169,6 @@ BlockDriver bdrv_cbw_filter = {
 BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   BlockDriverState *target,
                                   const char *filter_node_name,
-                                  uint64_t cluster_size,
                                   bool compress,
                                   BlockCopyState **bcs,
                                   Error **errp)
@@ -214,9 +213,8 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
     }
     appended = true;
 
-    state->cluster_size = cluster_size;
     state->bcs = block_copy_state_new(top->backing, state->target,
-                                      cluster_size, false, compress, errp);
+                                      false, compress, errp);
     if (!state->bcs) {
         error_prepend(errp, "Cannot create block-copy-state: ");
         goto fail;
