diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 9d349d61c68..fd506b4cb3c 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1067,7 +1067,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
         set_l2_entry(s, l2_slice, l2_index + i, offset | QCOW_OFLAG_COPIED);
 
         /* Update bitmap with the subclusters that were just written */
-        if (has_subclusters(s)) {
+        if (has_subclusters(s) && !m->prealloc) {
             uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
             unsigned written_from = m->cow_start.offset;
             unsigned written_to = m->cow_end.offset + m->cow_end.nb_bytes ?:
diff --git a/block/qcow2.c b/block/qcow2.c
index 54c9b7c1196..7c03d411705 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2096,6 +2096,7 @@ static coroutine_fn int qcow2_handle_l2meta(BlockDriverState *bs,
         QCowL2Meta *next;
 
         if (link_l2) {
+            assert(!l2meta->prealloc);
             ret = qcow2_alloc_cluster_link_l2(bs, l2meta);
             if (ret) {
                 goto out;
@@ -3130,6 +3131,7 @@ static int coroutine_fn preallocate_co(BlockDriverState *bs, uint64_t offset,
 
         while (meta) {
             QCowL2Meta *next = meta->next;
+            meta->prealloc = true;
 
             ret = qcow2_alloc_cluster_link_l2(bs, meta);
             if (ret < 0) {
@@ -4217,6 +4219,7 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         int64_t clusters_allocated;
         int64_t old_file_size, last_cluster, new_file_size;
         uint64_t nb_new_data_clusters, nb_new_l2_tables;
+        bool subclusters_need_allocation = false;
 
         /* With a data file, preallocation means just allocating the metadata
          * and forwarding the truncate request to the data file */
@@ -4298,6 +4301,8 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
                                    BDRV_REQ_ZERO_WRITE, NULL);
             if (ret >= 0) {
                 flags &= ~BDRV_REQ_ZERO_WRITE;
+                /* Ensure that we read zeroes and not backing file data */
+                subclusters_need_allocation = true;
             }
         } else {
             ret = -1;
@@ -4336,6 +4341,7 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
                     .offset       = nb_clusters << s->cluster_bits,
                     .nb_bytes     = 0,
                 },
+                .prealloc     = !subclusters_need_allocation,
             };
             qemu_co_queue_init(&allocation.dependent_requests);
 
diff --git a/block/qcow2.h b/block/qcow2.h
index 4ef4ae4ab0e..f3499e53bf4 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -463,6 +463,14 @@ typedef struct QCowL2Meta
      */
     bool skip_cow;
 
+    /**
+     * Indicates that this is not a normal write request but a preallocation.
+     * If the image has extended L2 entries this means that no new individual
+     * subclusters will be marked as allocated in the L2 bitmap (but any
+     * existing contents of that bitmap will be kept).
+     */
+    bool prealloc;
+
     /**
      * The I/O vector with the data from the actual guest write request.
      * If non-NULL, this is meant to be merged together with the data