Bennett, RobertBryant, Kelvin S.Sussman, AlanDas, RajaSaltz, JoelThere has been a great deal of recent interest in parallel I/O. This paper discusses issues in the design and implementation of a portable I/O library designed to optimize the performance of multiprocessor architectures that include multiple disks or disk arrays. The major emphasis of the paper is on optimizations that are made possible by the use of collective I/O, so that I/O requests for multiple processors can be combined to improve performance. Performance measurements from benchmarking our implementation of an I/O library that currently performs collective local optimizations, called Jovian, on three application templates are also presented.en-USTechnical Report