Performance of the Virgo interferometer longitudinal control system during the second science run

The second science run of the Virgo gravitational wave interferometer took place between July 2009 and January 2010. This paper describes the performance of the interferometer longitudinal control system in terms of duty cycle, stability and control noise. A science data taking duty cycle of about 80% was obtained over the six month run. Control noise was not limiting the detector sensitivity at any frequency. A discussion of observed thermal effects in the detector operation is also included.