Our goal with this paper is to contribute a common theoretical framework for studying the performance of disk-storage devices. Understanding the performance behavior of these devices will allow the prediction of the I/O cost in modern applications. Current disk technologies di er in terms of the fundamental modeling characteristics which include the magnetic/optical nature, the angular and linear velocities, the storage capacities and the transfer rates. Angular and linear velocities, storage capacities and transfer rates are made constant or variable in di erent existing disk products. Related work in this area has studied Constant Angular Velocity (CAV) magnetic disks and Constant Linear Velocity (CLV) optical disks. In this work, we present a comprehensive analytical model, validated through simulations, for the random retrieval performance of disk devices which takes into account all the above-mentioned fundamental characteristics and includes as special cases all the known disk-s...