A system for adaptive disk rearrangement

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Abstract
The speed of mass storage devices has a significant impact on the performance of computer systems. The speed that is realized on a particular mass storage device, however, depends heavily on how that device is used. Operating systems, such as the UNIX† time‐sharing system, use layout policies and head‐scheduling disciplines that are designed to work well on average. Numerous studies have shown that disk access patterns exhibit a high degree of locality. Further, studies have shown that these access patterns do not necessarily correspond to the usage patterns anticipated by the system's designers, and that head scheduling is used infrequently enough that it has limited effect. This paper describes the design, implementation, and use of a disk subsystem that adaptively corrects the disparity between expected access patterns and actual access patterns by reorganizing disk data. A representative experiment that demonstrates the resulting performance improvement is presented.

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