diff --git a/block.c b/block.c
index a307c151a8dbb..0af890f64759f 100644
--- a/block.c
+++ b/block.c
@@ -6480,6 +6480,13 @@ 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 are 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 c51388e99d8de..b48cd9ce6365d 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 e15395f2cbb75..df5ffc8d09e84 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;