diff --git a/block/raw-posix.c b/block/raw-posix.c
index 657af95..8b6b926 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -345,11 +345,20 @@ static int raw_reopen_prepare(BDRVReopenState *state,
 
     raw_s->fd = -1;
 
-    int fcntl_flags = O_APPEND | O_ASYNC | O_NONBLOCK;
+    int fcntl_flags = O_APPEND | O_NONBLOCK;
 #ifdef O_NOATIME
     fcntl_flags |= O_NOATIME;
 #endif
 
+#ifdef O_ASYNC
+    /* Not all operating systems have O_ASYNC, and those that don't
+     * will not let us track the state into raw_s->open_flags (typically
+     * you achieve the same effect with an ioctl, for example I_SETSIG
+     * on Solaris). But we do not use O_ASYNC, so that's fine.
+     */
+    assert((s->open_flags & O_ASYNC) == 0);
+#endif
+
     if ((raw_s->open_flags & ~fcntl_flags) == (s->open_flags & ~fcntl_flags)) {
         /* dup the original fd */
         /* TODO: use qemu fcntl wrapper */