See templator_front.org

* Text back end
  As the size of the dataset should be extensible, the simplest
  solution is to use one file for each sparse data set, and store a
  the name of this file in the group.
  Each integral can be a line in the file:
  i  j  k  l  x
  which can be read with "%10ld %10ld %10ld %10ld %24.16e".
  The offset can be used with ~fseek(69L*offset, SEEK_SET)~
  
* HDF5 Back end

  We need to declare the number of rows of the dataset as
  ~UNLIMITED~. This requires to use the ~Chunked~ storage, and the
  chunks should absolutely not be larger than 1MB.

  To extend the storage, see :
  https://support.hdfgroup.org/HDF5/doc1.6/UG/10_Datasets.html
  (figure 17)

  If the offset+num > nmax, we need to extend the dataset.