Applications using MPI-IO are still typically run on top of a parallel file system. The MPI-IO API is, in simple terms, intended to create I/O workloads that should perform well on top of the POSIX I/O interface (e.g. large, sequential I/O). But my understanding is that MPI-IO can have some major drawbacks, like a huge amount of compute-side network utilization used to reorganize and stage I/O.