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.