Optimal anticipatory position of a disk arm for queries of random length and location

The authors wish to identify the best anticipatory position for an idle disk arm which will minimize the expected distance travelled to a file segment of random length and location. Previous work focused primarily on requests of zero length, where the optimal position is the median of the requests’ location distribution. However in many scientific, geographical and multi-media database systems, it is typical that large objects must be fetched from disk and displayed in ‘real time’. We address such scenarios assuming that after dealing with a request the disk arm usually has sufficient time to return to an anticipatory position. They discuss systems where reads are always from left to right, as well as those in which reads can be performed in both directions (and hence the arm can start at the closer end to its anticipatory position). General analyses and instructive examples are provided.