In addition to the hints that you specify for a file in calls to ncmpi_create() and ncmpi_open(), you can use the PNETCDF_HINTS environment variable to pass hints that PnetCDF can use to tune I/O performance. For example:

$ export PNETCDF_HINTS="romio_cb_write=enable;romio_ds_write=disable;nc_header_align_size=262144"

Note that each hint is of the form "hint=val", and the hints are separated from one another by a ;. Additionally, hints can be PnetCDF or MPI-IO hints. In this example, romio_ prefixed hints are MPI-IO specific, whereas the nc_ prefix indicates PnetCDF hints.