I found this information in kb54528,not sure whether it helps :
When the Celerra with the primary Usermapper server is down, secondary Usermapper servers will use their cache to map existing users/groups. However, any requests to map new users/groups can only be handled by the primary Usermapper. If the primary is not available, new users/groups will not get mapped and will thus be unable to access CIFS shares. Existing users may experience some delay in response or intermittent timeouts if when have new users/groups try to access CIFS shares when primary Usermapper is inaccessible.