If a large parallel job running on 640 PNs reads from/writes to one disk installed in a PN, each PN accesses to the disk in sequence and performance degrades terribly. Although local I/O in which each PN reads from or writes to its own disk solves the problem, it is a very hard work to manage such a large number of partial files. Therefore, parallel I/O is greatly demanded in ES from the point of view of both performance and usability. The parallel file system (PFS) provides the parallel I/O features to ES (Figure 2). It enables handling multiple files, which are located on separate disks of multiple PNs, as logically one large file. Each process of a parallel program can read/write distributed data from/to the parallel file concurrently with one I/O statement to achieve high performance and usability of I/O.

Figure 2: Parallel File System (PFS): A parallel file, i.e. file on PFS is striped and stored cyclically in the specified blocking size into the disk of each PN. When a program accesses to the file, the File Access Library (FAL) sends a request for I/O via IN to the File Server on the node that owns the data to be accessed.