OPTIMAL ANTICIPATORY POSITION OF A DISK ARM FOR QUERIES OF RANDOM LENGTH AND LOCATION

We wish to identify the best anticipatory position for an idle disk ar m which will minimize the expected distance travelled to a file segmen t of random length and location. Previous work focused primarily on re quests of zero length, where the optimal position is the median of the requests' location distribution. However in many scientific, geograph ical and multi-media database systems, it is typical that large object s must be fetched from disk and displayed in 'real time'. We address s uch scenarios assuming that after dealing with a request the disk arm usually has sufficient time to return to an anticipatory position. We 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 th e arm can start at the closer end to its anticipatory position). Gener al analyses and instructive examples are provided.