EXHIBIT 2

JOINT FILING AGREEMENT

The undersigned hereby agree to the joint filing of the Amendment No. 16 to Schedule 13D to which this Agreement is attached.

Dated:  August 8, 2011

DISCOVERY GROUP I, LLC,

for itself and as general partner of

DISCOVERY EQUITY PARTNERS, L.P.

By   Michael R. Murphy*                                                        

        Michael R. Murphy

        Managing Member

Daniel J. Donoghue*                                                               

Daniel J. Donoghue

Michael R. Murphy*                                                               

Michael R. Murphy

*By: /s/ Mark Buckley                                                            

         Mark Buckley

         Attorney-in-Fact for Daniel J. Donoghue

         Attorney-in-Fact for Michael R. Murphy