BigFlowSim is a workflow I/O simulator that captures key implementation choices for remote I/O, including intensity, reuse, locality, access pattern, and data movement. With BigFlowSim, it is possible to generate synthetic benchmarks, and then perform sensitivity analyses to quantify the effects of the various I/O parameters. In addition to generating synthetic workflows, BigFlowSim is also able to replay traces (of I/O access) captured from execution of actual distributed scientific workflows.