RIO (originally ROSS-IO) is a checkpointing API for Rensselaer's Optimistic Simulation System. RIO is for checkpoint-restart operations, it cannot be used to created incremental checkpoints for fault tolerance.
The documentation for RIO can be found on the ROSS website (Look for the RIO section on the archive page). The documentation includes:
The full RIO API has been implemented in the PHOLD-IO model.