diff --git a/src/emqttd_sm.erl b/src/emqttd_sm.erl
index 5becbba1e..0d7ddf9c8 100644
--- a/src/emqttd_sm.erl
+++ b/src/emqttd_sm.erl
@@ -57,6 +57,7 @@
 %%%=============================================================================
 
 mnesia(boot) ->
+    %% global session...
     ok = emqttd_mnesia:create_table(session, [
                 {type, ordered_set},
                 {ram_copies, [node()]},