Using System-Level Models to Evaluate I/O Subsystem Designs

We describe a system-level simulation model and show that it enables accurate predictions of both I/O subsystem and overall system performance. In contrast, the conventional approach for evaluating the performance of an I/O subsystem design, which is based on standalone subsystem models, is often unable to accurately predict performance changes because it is too narrow in scope. In particular, conventional methodology treats all I/O requests equally, ignoring differences in how individual requests' response times affect system behavior (including both system performance and the subsequent I/O workload). We introduce the concept of request criticality to describe these feedback effects and show that real I/O workloads are not approximated well by either open or closed input models. Because conventional methodology ignores this fact, it often leads to inaccurate performance predictions and can thereby lead to incorrect conclusions and poor design choices. We illustrate these problems with real examples and show that a system-level model, which includes both the I/O subsystem and other important system components (e.g., CPUs and system software), properly captures the feedback and subsequent performance effects.