When the user ran "find" or "du -sh" command on the client, MDS dumps the many call traces and the error messages and the can't response to the clients. (MDS didn't hang and down, but the response was really slow).
I'm attaching the /var/log/messages and we can see many call traces on these log files. I'm not sure, all are caused by same problem, but at least the user ran several "find and "du -sh" commands on Jan 17 and Jan 18.