InfiniBand is becoming increasingly popular as a fast interconnect technology between servers and storage. It has far better price/performance ratio compared to both Gigabit Ethernet and 10 Gigabit Ethernet, and hence is being increasingly used for high-performance computing applications. PVFS2, the second generation Parallel Virtual File System (PVFS), is a distributed file system for parallel data access that is being increasingly used in clustered applications. As previous studies have shown that in general, PVFS2 over InfiniBand offers enhanced I/O rates compared to PVFS2 over TCP and Gigabit Ethernet. Apart from the hardware technology, the application programming interface into the file system also makes a difference. To get better parallel performance, the choice of a file system interface is important. Our study is to benchmark and compare the performance of PVFS2 running over InfiniBand using different file system interfaces. IOR is a popular I/O microbenchmarking tool that supports the POSIX and MPI I/O file system interfaces. In addition to testing these already supported interfaces, we have written a PVFS2 module extension for IOR to support native PVFS2 interfaces into the PVFS2 file system. As we shall see in this study, using native PVFS2 interface offers significant performance benefit compared to other file system interfaces on the PVFS2 file system. Our benchmarking effort also involves studying the effect of a multi-client environment on the I/O performance of different file system interfaces. Based on the benchmarking results we obtain, we determine the most efficient application programming interface for parallel I/O on PVFS2 in a typical multi-client parallel application scenario.