Past rsearch on heigh performance computers for scientific applications has concentrated on CPU speed and exploitation of pa parallelism, but has, until very recently, neglected I/O considerations. This paper presents a study of the production workfload at the San Diego Supercomputer Center from an 1/0 requiremnts and characteristic perspective, Results of our and analyses support our hypothesis that a significant proportion of 1/O , long running, frequently executid scientific applications have predicable I/O requirements.