diff --git a/blockdev.c b/blockdev.c
index 50421c8..62a4586 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2639,8 +2639,6 @@
     aio_context_release(aio_context);
 }
 
-#define DEFAULT_MIRROR_BUF_SIZE   (10 << 20)
-
 void qmp_drive_mirror(const char *device, const char *target,
                       bool has_format, const char *format,
                       bool has_node_name, const char *node_name,
@@ -2682,7 +2680,7 @@
         granularity = 0;
     }
     if (!has_buf_size) {
-        buf_size = DEFAULT_MIRROR_BUF_SIZE;
+        buf_size = 0;
     }
     if (!has_unmap) {
         unmap = true;