An introduction to disk drive modeling

Abstract

Although disk storage densities are improving impressively (60% to 130% compounded annually), performance improvements have been occurring at only about 7% to 10% compounded annually over the last decade. As a result, disk system performance is fast becoming a dominant factor in overall svstem behavior. Naturally, researchers want to improve overall I/O performance, of which a large component is the performance of the disk drive itself. This research often involves using analytical or simulation models to compare alternative approaches, and the quality of these models determines the quality of the conclusions: indeed, the wrong modeling assumptions can lead to erroneous conclusions. Nevertheless, little work has been done to develop or describe accurate disk drive models. This may explain the commonplace use of simple, relatively inaccurate models. We believe there is much room for improvement. This article demonstrates and describes a calibrated, high-quality disk drive model in which the overall error factor is 14 times smaller than that of a simple first-order model. We describe the various disk drive performance components separately, then show how their inclusion improves the simulation model. This enables an informed trade-off between effort and accuracy. In addition, we provide detailed characteristics for two disk drives, as well as a brief description of a simulation environment that uses the disk drive model.