This paper discusses the design of the file system for DEMOS, an operating system being developed for the CRAY-1 computer at Los Alamos Scientific Laboratory. The goals to be met, in particular the performance and usability considerations, are outlined. A description is given of the user interface and the general structure of the file system and the file system routines. A simple model of program behavior is used to demonstrate the effect of buffering data by the file system routines. A disk space allocation strategy is described which will take advantage of this buffering. The last section outlines how the performance mechanisms are integrated into the file system routines.