The throughput of a disk subsystem is critical to the performance of a computer system. The 4.3BSD Unix operating system provides a good example for looking at the heuristics that are used to optimize the positioning of the read-write heads of a disk drive. In this paper we investigate the heuristics used in the BSD hp disk driver. These heuristics are driven by three parameters that can be changed by the system manager. We investigate the interaction of these parameters with the BSD fast filesystem. This provides a way to see the effects of the BSD fast filesystem. Finally we present the results of monitoring three active filesystems during day-to-day use, to see what effect these heuristics can have.