...

This takes a second or two to complete.  This process can be scripted easily enough, and there is a template for this in the NMIS8 distribution in /path/to/nmis8/admin/rrd_file_update.pl, the script would need to determine the RRA numbers by running rrdtool info first and then execute the above rrdtool resize commands for each RRD file.

Where are NMIS RRD Files Stored?

NMIS stores the RRD files in the folder /path/to/nmis8/database, then in various sub-directories depending on what is required.  The script rrd_file_update.pl has the code to traverse a directory structure.  NMIS can easily be configured to store RRD files in another location, this is controlled by the NMIS config item database_root which by default is set to '<nmis_data>/database'.

Conclusion

NMIS8 is a powerful network management system which can keep as much data as you have disk space to store, the defaults make sense for most organisations and provide a good balance between statistical granularity and disk space.