6.33 Change between Collective and Independent Parallel Access: nc_var_par_access
The function nc_var_par_access changes whether read/write operations
on a parallel file system are performed collectively (the default) or
independently on the variable. This function can only be called if the
file was created with nc_create_par (see nc_create_par) or opened
with nc_open_par (see nc_open_par).
Calling this function affects only the open file - information about
whether a variable is to be accessed collectively or independently is
not written to the data file. Every time you open a file on a parallel
file system, all variables default to collective operations. The change
a variable to independent lasts only as long as that file is open.
The variable can be changed from collective to independent, and back,
as often as desired.
Usage
int nc_var_par_access(int ncid, int varid, int access);
ncid- NetCDF ID, from a previous call to nc_open_par (see nc_open_par) or
nc_create_par (see nc_create_par).
varid- Variable ID.
access- NC_INDEPENDENT to set this variable to independent
operations.NC_COLLECTIVE to set it to collective operations.
Return Values
NC_NOERR- No error.
Example
Here is an example using nc_var_par_access:
#include <netcdf.h>
...
int ncid, v1id, dimids[NDIMS];
int data[DIMSIZE*DIMSIZE], j, i, res;
...
/* Create a parallel netcdf-4 file. */
if ((res = nc_create_par(FILE, NC_MPIIO, comm, info, &ncid)))
BAIL(res);
/* Create two dimensions. */
if ((res = nc_def_dim(ncid, "d1", DIMSIZE, dimids)))
BAIL(res);
if ((res = nc_def_dim(ncid, "d2", DIMSIZE, &dimids[1])))
BAIL(res);
/* Create one var. */
if ((res = nc_def_var(ncid, "v1", NC_INT, NDIMS, dimids, &v1id)))
BAIL(res);
if ((res = nc_enddef(ncid)))
BAIL(res);
/* Tell HDF5 to use independent parallel access for this var. */
if ((res = nc_var_par_access(ncid, v1id, NC_INDEPENDENT)))
BAIL(res);
/* Write slabs of phony data. */
if ((res = nc_put_vara_int(ncid, v1id, start, count,
&data[mpi_rank*QTR_DATA])))
BAIL(res);
