I/O system performance has become and will remain an important research focus as processor speeds continue to improve faster than I/O system speeds. Because there presently exists no accurate way to measure I/O system performance, it is hard to objectively evaluate new I/O system designs. A measure for I/O performance that simulates measured system workloads is being developed. The few studies of I/O system workloads to date have failed to consider: (i) mixtures of both file system activity and paging activity, (ii) CPU overhead of I/O system calls, (iii) variations of I/O activity across different application domains (e.g. business vs. scientific applications), and (iv) interarrival timings of I/O requests. Detailed measurements of these factors in I/O system performance, will be made and used to construct a comprehensive measure of I/O system performance. This measure can be used to compare I/O system designs and to balance processor performance with I/O system performance. Preliminary measurements indicate that a good balance between processor and I/O performance is currently rarely achieved. Results for these studies will make it possible to analyze performance tradeoffs for new I/O system designs.