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The Computer Journal 1976 19(4):301-305; doi:10.1093/comjnl/19.4.301
© 1976 by British Computer Society
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Seek times for disc file processing: some results from probability theory

V. Siskind1 * and J. Rosenhead2

1 University of Queensland, Brisbane, Australia, 2 London School of Economics, Houghton Street, London, UK

This paper considers the duration of seek times for a skip-serially processed file on a moveable head disc pack. It is shown that under certain assumptions about the generation of requests for records, the distributions of both the number of records to be processed and the number of distinct cylinders to be accessed per run are binomial. Probabilistic arguments from the theory of order statistics are used to derive the distribution of the number of cylinders separating consecutive cylinders accessed. A general functional form is introduced for the relation between inter-cylinder distance and duration of seek time, and used to derive a general expression for the expectation of total seek time for a scan of the file. Results are computed for a numerical example. These show, first, the advantage of software which ensures that successive scans operate in opposite directions; and second, the danger of serious inaccuracy (especially at low hit-ratios) if simple approximations are used based on the value of the seek time function at the mean inter-cylinder distance.

Received December 1975.

* Now at: University of Queensland, Brisbane, Australia.

§ London School of Economics, Houghton Street, London WC2A 2AE


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