The MPI-IO subroutines are only available with the MPI threads library.
MPI-IO communication will not operate if the MP_SINGLE_THREAD environment variable is set to yes. In the threads library, a call to MPI_INIT with MP_SINGLE_THREAD set to yes is equivalent to what might be expected with a call to MPI_INIT_THREAD specifying MPI_THREAD_FUNNELED. A call with MP_SINGLE_THREAD set to no is equivalent to using MPI_THREAD_MULTIPLE. The default setting of MP_SINGLE_THREAD is no, so the default behavior of the threads library is MPI_THREAD_MULTIPLE. (Note that in PE MPI, thread behavior is determined before calling MPI_INIT or MPI_INIT_THREAD. A call to MPI_INIT_THREAD with MPI_THREAD_FUNNELED will not actually mimic MP_SINGLE_THREAD.)
MPI-IO is intended to be used with the IBM General Parallel File System (GPFS) for production use. File access through MPI-IO normally requires that a single GPFS file system image be available across all tasks of an MPI job. Shared file systems such as AFS(R) and NFS do not meet this requirement when used across multiple nodes. PE MPI-IO can be used for program development on any other file system that supports a POSIX interface (AFS, DFS(TM), JFS, or NFS) as long as all tasks run on a single node or workstation, but this is not expected to be a useful model for production use of MPI-IO.
In MPI-IO, which tasks perform I/O is not determined by which tasks make MPI-IO calls. By default, MPI-IO performs I/O through an agent at each task of the job. I/O agents can be restricted to specific nodes by using an I/O node file. This should be done any time there is not a single GPFS file system available to all nodes on which tasks are to run. PE MPI-IO can be used without all tasks having access to a single file system image by using the MP_IONODEFILE environment variable. See IBM Parallel Environment for AIX: Operation and Use, Volume 1 for information about MP_IONODEFILE.