• The chunking may be set for each variable after it is defined.
• The default chunking in the 4.0 release was not good for very large variables. In the 4.0.1 release we have a more complex method of selecting default chunksizes for each dimension. In 4.1.1 we changed it again:

     /* Pick a chunk length for each dimension. */
     for (d = 0; d < var->ndims; d++)
        if (var->dim[d]->unlimited)
  	 var->chunksizes[d] = 1;
        else
        {
  	 var->chunksizes[d] = (pow((double)DEFAULT_CHUNK_SIZE/(num_values * type_size), 
  				   1/(double)(var->ndims - num_unlim)) * var->dim[d]->len + .5);
  	 if (var->chunksizes[d] > var->dim[d]->len)
  	    var->chunksizes[d] = var->dim[d]->len;
        }
  
  

• Set the chunking with the nc_def_var_chunking function:

  
  /* Define chunking for a variable. This must be done after nc_def_var
     and before nc_enddef. */
  EXTERNL int
  nc_def_var_chunking(int ncid, int varid, int *chunkalgp, 
  		    int *chunksizesp, int *extend_incrementsp);
  
  /* Inq chunking stuff for a var. */
  EXTERNL int
  nc_inq_var_chunking(int ncid, int varid, int *chunkalgp, 
  		    int *chunksizesp, int *extend_incrementsp);
  
  

2010 Unidata NetCDF Workshop > Chunking and Deflating Data with NetCDF-4