• The chuncking 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:

  /* These are limits for default chunksizes. (2^16 and 2^20). */
  #define NC_LEN_TOO_BIG 65536 
  #define NC_LEN_WAY_TOO_BIG 1048576
  
  /* Now we must determine the default chunksize. */
   if (dim->unlimited)
      chunksize[d] = 1;
   else if (dim->len < NC_LEN_TOO_BIG)
      chunksize[d] = dim->len;
   else if (dim->len > NC_LEN_TOO_BIG && dim->len <= NC_LEN_WAY_TOO_BIG)
      chunksize[d] = dim->len / 2 + 1;
   else
      chunksize[d] = NC_LEN_WAY_TOO_BIG;
  

• 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);
  
  

2009 Unidata NetCDF Workshop for Developers and Data Providers > Chunking and Deflating Data with NetCDF-4