When a RWN is booked, HBOOK reserves space in memory in which to store the data, as specified by the NWBUFF argument of routine HBOOKN. As the RWN is filled by a call to HFN, this space will be successively used until full. At that time, a new memory allocation will be made and the process continues.
If this data is then written to disk, it appears as one enormous logical record. In addition, it most cases the last block of data will not be completely filled thus wasting space.
If one tries to reprocess this RWN at a later date, e.g. with PAW, there must be enough space in memory to store the entire Ntuple. This often gives rise to problems and so the following alternative method is recommended.