EXHIBIT 2

JOINT FILING AGREEMENT

The undersigned hereby agree to the joint filing of the Amendment No. 15 to Schedule 13D to which this Agreement is attached.

Dated:  July 27, 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