diff --git a/block/mirror.c b/block/mirror.c
index fc6b9b7..896972c 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -207,7 +207,7 @@ static void coroutine_fn mirror_run(void *opaque)
     if (backing_filename[0] && !s->target->backing_hd) {
         bdrv_get_info(s->target, &bdi);
         if (s->granularity < bdi.cluster_size) {
-            s->buf_size = bdi.cluster_size;
+            s->buf_size = MAX(s->buf_size, bdi.cluster_size);
             length = (bdrv_getlength(bs) + s->granularity - 1) / s->granularity;
             s->cow_bitmap = bitmap_new(length);
         }
@@ -416,8 +416,8 @@ static BlockJobType mirror_job_type = {
 };
 
 void mirror_start(BlockDriverState *bs, BlockDriverState *target,
-                  int64_t speed, int64_t granularity, MirrorSyncMode mode,
-                  BlockdevOnError on_source_error,
+                  int64_t speed, int64_t granularity, int64_t buf_size,
+                  MirrorSyncMode mode, BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   BlockDriverCompletionFunc *cb,
                   void *opaque, Error **errp)
@@ -455,7 +455,7 @@ void mirror_start(BlockDriverState *bs, BlockDriverState *target,
     s->target = target;
     s->mode = mode;
     s->granularity = granularity;
-    s->buf_size = granularity;
+    s->buf_size = MAX(buf_size, granularity);
 
     bdrv_set_dirty_tracking(bs, granularity);
     bdrv_set_enable_write_cache(s->target, true);
diff --git a/blockdev.c b/blockdev.c
index 07fd327..ad25b9b 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1188,12 +1188,15 @@ void qmp_block_commit(const char *device,
     drive_get_ref(drive_get_by_blockdev(bs));
 }
 
+#define DEFAULT_MIRROR_BUF_SIZE   (10 << 20)
+
 void qmp_drive_mirror(const char *device, const char *target,
                       bool has_format, const char *format,
                       enum MirrorSyncMode sync,
                       bool has_mode, enum NewImageMode mode,
                       bool has_speed, int64_t speed,
                       bool has_granularity, uint32_t granularity,
+                      bool has_buf_size, int64_t buf_size,
                       bool has_on_source_error, BlockdevOnError on_source_error,
                       bool has_on_target_error, BlockdevOnError on_target_error,
                       Error **errp)
@@ -1222,6 +1225,10 @@ void qmp_drive_mirror(const char *device, const char *target,
     if (!has_granularity) {
         granularity = 0;
     }
+    if (!has_buf_size) {
+        buf_size = DEFAULT_MIRROR_BUF_SIZE;
+    }
+
     if (granularity != 0 && (granularity < 512 || granularity > 1048576 * 64)) {
         error_set(errp, QERR_INVALID_PARAMETER, device);
         return;
@@ -1311,7 +1318,7 @@ void qmp_drive_mirror(const char *device, const char *target,
         return;
     }
 
-    mirror_start(bs, target_bs, speed, granularity, sync,
+    mirror_start(bs, target_bs, speed, granularity, buf_size, sync,
                  on_source_error, on_target_error,
                  block_job_cb, bs, &local_err);
     if (local_err != NULL) {
diff --git a/hmp.c b/hmp.c
index 0f3347d..99fd892 100644
--- a/hmp.c
+++ b/hmp.c
@@ -796,7 +796,7 @@ void hmp_drive_mirror(Monitor *mon, const QDict *qdict)
 
     qmp_drive_mirror(device, filename, !!format, format,
                      full ? MIRROR_SYNC_MODE_FULL : MIRROR_SYNC_MODE_TOP,
-                     true, mode, false, 0, false, 0,
+                     true, mode, false, 0, false, 0, false, 0,
                      false, 0, false, 0, &errp);
     hmp_handle_error(mon, &errp);
 }
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 1165339..f7279b9 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -345,6 +345,7 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
  * @target: Block device to write to.
  * @speed: The maximum speed, in bytes per second, or 0 for unlimited.
  * @granularity: The chosen granularity for the dirty bitmap.
+ * @buf_size: The amount of data that can be in flight at one time.
  * @mode: Whether to collapse all images in the chain to the target.
  * @on_source_error: The action to take upon error reading from the source.
  * @on_target_error: The action to take upon error writing to the target.
@@ -358,8 +359,8 @@ void commit_start(BlockDriverState *bs, BlockDriverState *base,
  * @bs will be switched to read from @target.
  */
 void mirror_start(BlockDriverState *bs, BlockDriverState *target,
-                  int64_t speed, int64_t granularity, MirrorSyncMode mode,
-                  BlockdevOnError on_source_error,
+                  int64_t speed, int64_t granularity, int64_t buf_size,
+                  MirrorSyncMode mode, BlockdevOnError on_source_error,
                   BlockdevOnError on_target_error,
                   BlockDriverCompletionFunc *cb,
                   void *opaque, Error **errp);
diff --git a/qapi-schema.json b/qapi-schema.json
index 7ab11f8..6bc0a87 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -1687,6 +1687,9 @@
 #               are smaller than that, else the cluster size.  Must be a
 #               power of 2 between 512 and 64M (since 1.4).
 #
+# @buf-size: #optional maximum amount of data in flight from source to
+#            target (since 1.4).
+#
 # @on-source-error: #optional the action to take on an error on the source,
 #                   default 'report'.  'stop' and 'enospc' can only be used
 #                   if the block device supports io-status (see BlockInfo).
@@ -1704,7 +1707,7 @@
   'data': { 'device': 'str', 'target': 'str', '*format': 'str',
             'sync': 'MirrorSyncMode', '*mode': 'NewImageMode',
             '*speed': 'int', '*granularity': 'uint32',
-            '*on-source-error': 'BlockdevOnError',
+            '*buf-size': 'int', '*on-source-error': 'BlockdevOnError',
             '*on-target-error': 'BlockdevOnError' } }
 
 ##
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 835ea26..273b4a6 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -939,7 +939,7 @@ EQMP
         .name       = "drive-mirror",
         .args_type  = "sync:s,device:B,target:s,speed:i?,mode:s?,format:s?,"
                       "on-source-error:s?,on-target-error:s?,"
-                      "granularity:i?",
+                      "granularity:i?,buf-size:i?",
         .mhandler.cmd_new = qmp_marshal_input_drive_mirror,
     },
 
@@ -964,6 +964,8 @@ Arguments:
 - "speed": maximum speed of the streaming job, in bytes per second
   (json-int)
 - "granularity": granularity of the dirty bitmap, in bytes (json-int, optional)
+- "buf_size": maximum amount of data in flight from source to target, in bytes
+  (json-int, default 10M)
 - "sync": what parts of the disk image should be copied to the destination;
   possibilities include "full" for all the disk, "top" for only the sectors
   allocated in the topmost image, or "none" to only replicate new I/O
diff --git a/tests/qemu-iotests/041 b/tests/qemu-iotests/041
index a1299b3..b040820 100755
--- a/tests/qemu-iotests/041
+++ b/tests/qemu-iotests/041
@@ -207,6 +207,37 @@ class TestSingleDrive(ImageMirroringTestCase):
         self.assertTrue(self.compare_images(test_img, target_img),
                         'target image does not match source after mirroring')
 
+    def test_small_buffer(self):
+        self.assert_no_active_mirrors()
+
+        # A small buffer is rounded up automatically
+        result = self.vm.qmp('drive-mirror', device='drive0', sync='full',
+                             buf_size=4096, target=target_img)
+        self.assert_qmp(result, 'return', {})
+
+        self.complete_and_wait()
+        result = self.vm.qmp('query-block')
+        self.assert_qmp(result, 'return[0]/inserted/file', target_img)
+        self.vm.shutdown()
+        self.assertTrue(self.compare_images(test_img, target_img),
+                        'target image does not match source after mirroring')
+
+    def test_small_buffer2(self):
+        self.assert_no_active_mirrors()
+
+        qemu_img('create', '-f', iotests.imgfmt, '-o', 'cluster_size=%d,size=%d'
+                        % (TestSingleDrive.image_len, TestSingleDrive.image_len), target_img)
+        result = self.vm.qmp('drive-mirror', device='drive0', sync='full',
+                             buf_size=65536, mode='existing', target=target_img)
+        self.assert_qmp(result, 'return', {})
+
+        self.complete_and_wait()
+        result = self.vm.qmp('query-block')
+        self.assert_qmp(result, 'return[0]/inserted/file', target_img)
+        self.vm.shutdown()
+        self.assertTrue(self.compare_images(test_img, target_img),
+                        'target image does not match source after mirroring')
+
     def test_large_cluster(self):
         self.assert_no_active_mirrors()
 
diff --git a/tests/qemu-iotests/041.out b/tests/qemu-iotests/041.out
index 3a89159..84bfd63 100644
--- a/tests/qemu-iotests/041.out
+++ b/tests/qemu-iotests/041.out
@@ -1,5 +1,5 @@
-....................
+......................
 ----------------------------------------------------------------------
-Ran 20 tests
+Ran 22 tests
 
 OK
