Three methods (seek overlap, seek arm scheduling, static file repositioning) for improvement of disk subsystem performance are reviewed. Detailed measured data are reported for seek time, probability of zero-length seek, latency, and transfer time, for a 12-spindle CDC 844-41 disk subsystem shared between two CDC Cyber 170/750 central processors. The probability of zero-length seeks is shown to be high, and the spindle queue lengths are observed to be low. The transfer time data are very different from the data published by others for IBM systems. A detailed simulation model of the measured system is shown to validate. This model is then used to demonstrate that seek arm scheduling is unlikely to produce much improvement, that static file repositioning can improve performance by about 20%, and that seek overlap can almost double the interactive capacity of the system measured.
