The vesta parallel file system is intended to solve the I/O problems of massively parallel multicomputers executing numericallly intensive scientific applications. It provides parallel access from the applicatins fo files distributed across the multicomputer's low-latency network. The vesta interface provides a user-defined parallel view of file data. which fives users some control over the layout of data. THis is useful for tailoring data layout to match common access patterns. The interface also allows user-fdefined partitionsing and repartitioning for file swithout moving data amonong storage nodes. Libraries with higher-level interfaces that hide the layout details, while exploiting the power of parallel access, may be implemented above th basic interface. wi show how collective I/O operations can be implemented . and define six parallel access modes to vesta files. Each mode has unique characteristics in terms of how the processes share the file and how their accesses are interleaved. The combination of user-defined file partitioning and the six acess modes gives users very versatile parallel file access.