gsizes[0] = m;  /* no. of rows in global array */
gsizes[1] = n;  /* no. of columns in global array*/

psizes[0] = 2;  /* no. of processes in vertical dimension 
                   of process grid */
psizes[1] = 3;  /* no. of processes in horizontal dimension 
                   of process grid */

lsizes[0] = m/psizes[0];   /* no. of rows in local array */
lsizes[1] = n/psizes[1];   /* no. of columns in local array */

dims[0] = 2;
dims[1] = 3;
periods[0] = periods[1] = 1;
MPI_Cart_create(MPI_COMM_WORLD, 2, dims, periods, 0, &comm);
MPI_Comm_rank(comm, &rank);
MPI_Cart_coords(comm, rank, 2, coords);

/* global indices of the first element of the local array */
start_indices[0] = coords[0] * lsizes[0];
start_indices[1] = coords[1] * lsizes[1];

MPI_Type_create_subarray(2, gsizes, lsizes, start_indices, 
                         MPI_ORDER_C, MPI_FLOAT, &filetype);
MPI_Type_commit(&filetype);

MPI_File_open(comm, "/pfs/datafile", 
              MPI_MODE_CREATE | MPI_MODE_WRONLY, 
	      MPI_INFO_NULL, &fh);
MPI_File_set_view(fh, 0, MPI_FLOAT, filetype, "native", 
		  MPI_INFO_NULL);

local_array_size = lsizes[0] * lsizes[1];
MPI_File_write_all(fh, local_array, local_array_size, 
		   MPI_FLOAT, &status);

MPI_File_close(&fh);