This paper introduces the novel I/O benchmark PRIOmark that measures file system and disk I/O performance of modern computer systems. It characterizes the file system performance of single and distributed systems by means of two different file system interfaces. Disk I/O performance is measured using the operating system specific rawdevices. PRIOmark features the possibility of a complex workload definition in order to specify workloads for many different application domains. The output format simplifies comparability of benchmark results of different architectures. Additionally, the paper exemplarily illustrates the use of PRIOmark by comparing the file system performance of three file systems (NFS, GFS and PVFS) on a cluster with 12 processors.