Many computer systems are operating significantly below the potential of their central processors because of inadequate support from the auxiliary storage units. Most storage units, e.g. drums and disks, store the information on rotating surfaces, and the delays associated with retrieving information from these devices are substantial; access times on the order of ten to one hundred milliseconds are not uncommon, while many central processors are capable of executing well over a million operations per second.Unless unusual care is taken in the organization and scheduling of these rotating storage units, they will become the dominant bottleneck in the computer system.