The process-flow model: examining I/O performance from the system's point of view

Input/output subsystem performance is currently receiving considerable research attention. Significant effort has been focused on reducing average I/O response times and increasing throughput for a given workload. This work has resulted in tremendous advances in I/O subsystem performance. It is unclear, however, how these improvements will be reflected in overall system performance. The central problem lies in the fact that the current method of study tends to treat all I/O requests aa equally important. We introduce a three class taxonomy of I/O requests based on their effects on system performance. We denote the three classes time-critical, time-limited, and time-noncritical. A system-level, trace-driven simulation model has been developed for the purpose of studying disk scheduling algorithms. By incorporating knowledge of I/O classes, algorithms tuned for system performance rather than I/O subsystem performance may be developed. Traditional I/O subsystem simulators would rate such algorithms unfavorably because they produce suboptimal subsystem performance. By studying the I/O subsystem via global, system-level simulation, one can more easily identify changes that will improve overall system performance.