|Abstract||MPI-IO is a standard, portable interface for parallel file I/O that was de- fined as part the MPI-2 (Message Passing Interface) Standard in 1997. It can be used either directly by application programmers or by writers of high-level libraries as an interface for portable, high-performance I/O in parallel pro- grams. It has many features specifically designed to efficiently support the I/O needs of parallel scientific applications. MPI-IO is an interface that sits above a parallel file system and below an application or a high-level I/O library, as illustrated in Figure 13.1. Hence it is often referred to as “middleware” for parallel I/O.