1. An apparatus for simulating a parallel supercomputing cluster having a system of compute nodes and an I/O subsystem connected to the compute nodes for storing checkpoint data from the compute nodes, the apparatus comprising a digital computer including a data processor and non-transitory computer readable storage medium storing a simulation program, the simulation program including a model of the system of compute nodes and a model of the I/O subsystem and also including computer instructions that, when executed by the data processor, perform the steps of: (a) receiving input parameters defining the system of compute nodes and input parameters defining the I/O subsystem; (b) computing a total number of computational floating point operations for a time between checkpoints for different configurations of the parallel supercomputing cluster, and computing an amount of disk storage required to store the checkpoint data for the different configurations of the parallel supercomputing cluster; and (c) presenting, to a user, a summary of the computed total number of computational floating point operations for the time between checkpoints for the different configurations of the parallel supercomputing cluster, and presenting, to the user, a summary of the computed amount of disk storage required to store the checkpoint data for the different configurations of the parallel supercomputing cluster.