Exascale supercomputers will have the potential for billion-way parallelism. While physical implementations of these systems are currently not available, HPC system designers can develop models of exascale systems to evaluate system design points. Modeling these systems and associated subsystems is a significant challenge. In this paper, we present the Co-design of Exascale Storage System (CODES) framework for evaluating exascale storage system design points. As part of our early work with CODES, we discuss the use of the CODES framework to simulate leadership-scale storage systems in a tractable amount of time using parallel discrete-event simulation. We describe the current storage system models and protocols included with the CODES framework and demonstrate the use of CODES through simulations of an existing petascale storage system.