NCL Home > Documentation > Functions > File IO

filechunkdimdef

Defines chunking dimension names, dimension sizes, unlimited dimensions on a supported file.

Prototype

	procedure filechunkdimdef (
		thefile       [1] : file,             
		dim_names     [*] : string,           
		dim_sizes     [*] : integer or long,  
		dim_unlimited [*] : logical           
	)

Arguments

thefile

The reference to the file that you want to write to with chunking. This reference must be created by the addfile function.

dim_names

An array of chunking dimension names you want to write to thefile.

dim_sizes

An array of dimension sizes of the chunking dimensions you want to use thefile.

As of version 6.0.0, this can be of type long, allowing dimension sizes greater than or equal to 2 gigabytes (GB) on 64-bit systems. But it is usually a fraction of the dimension size, such as half, quauter, etc.

dim_unlimited

This parameter does not apply for chunking, but we keep it here to make it has same argument-list as filedimdef.

If there is unlimited dimension, it will be a good practice to set the leftmost chunking dimension to 1 (if it is a multi-dimension variable), as it is hard to predict how big this dimension will be.

Description

The filechunkdimdef procedure is used to pre-define chunking dimensions in a file. The supported file must have been opened as either read/write ("w") or create ("c").

Using this procedure is not to save writing time, but to save reading time, if later the variable is frequently partially read.

See Also

filedimdef, filevarchunkdef

Examples

Write dimensions "lon", "lat", "lev, and "time" to a netCDF file, with "time" being an unlimited dimension, then use chunk options to write variables in chunking.

  setfileoption("nc", "Format",  "NetCDF4Classic")

  ncf = addfile("myfile.nc","c")
;
; Define dimensions.
;
  nlon =  64
  nlat = 128
  nlev =  10
  ntim =  -1

  dim_names = (/ "lon",  "lat",  "lev", "time" /)
  dim_sizes = (/ nlon ,  nlat ,  nlev ,  ntim  /)
  dimUnlim = (/ False , False,  False ,  True  /)
  filedimdef( ncf, dim_names, dim_sizes, dimUnlim )

  mtim = 1
  mlat = nlat/2
  mlon = nlon/2
  mlev = nlev/2
  chunkSizes = (/ mtim, mlev, mlat, mlon /)
  dimUnlim(0) = False
  filechunkdimdef(fout,dimNames,chunkSizes,dimUnlim)
"ncdump myfile.nc" produces the following output:
netcdf myfile {
dimensions:
        lon = 64 ;
        lat = 128 ;
        lev = 10 ;
        time = UNLIMITED ; // (0 currently)
}