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Modern microprocessor technology is advancing at an incredible rate, and speedups of 40 to 60 percent
compounded annually have become the norm. Although disk storage densities are also improving
impressively (60 to 80 percent compounded annually), performance improvements have been occurring at
only about 7 to 10 percent compounded annually over the last decade. As a result, disk system performance
is fast becoming a dominant factor in overall system behavior.
Naturally, researchers want to improve overall I/O performance, of which a large component is the
performance of the disk drive itself. This research often involves using analytical or simulation models to
compare alternative approaches, and the quality of these models determines the quality of the conclusions;
indeed, the wrong modeling assumptions can lead to erroneous conclusions. Nevertheless, little work has
been done to develop or describe accurate disk drive models. This may explain the commonplace use of
simple, relatively inaccurate models.
We believe there is much room for improvement. This article demonstrates and describes a calibrated, high-
quality disk drive model in which the overall error factor is 14 times smaller than that of a simple first-order
model. We describe the various disk drive performance components separately, then show how their
inclusion improves the simulation model. This enables an informed trade-off between effort and accuracy.
In addition, we provide detailed characteristics for two disk drives, as well as a brief description of a
simulation environment that uses the disk drive model.
Characteristics of modern disk drives
To model disk drives, we must understand how they behave. Thus, we begin with an overview of the current
state of the art in nonremovable magnetic disk drives with embedded SCSI (Small Computer Systems
Interconnect) controllers, since these are widely available.
Disk drives contain a mechanism and a controller. The mechanism is made up of the recording components
(the rotating disks and the heads that access them) and the positioning components (an arm assembly that
moves the heads into the correct position together with a track-following system that keeps it in place). The
disk controller contains a microprocessor, some buffer memory, and an interface to the SCSI bus. The
controller manages the storage and retrieval of data to and from the mechanism and performs mappings
between incoming logical addresses and the physical disk sectors that store the information.
Below, we look more closely at each of these elements, emphasizing features that need to be considered
when creating a disk drive model. It will become clear that not all these features are equally important to a
model’s accuracy.
The recording components. Modern disks range in size from 1.3 to 8 inches in diameter; 2.5, 3.5, and 5.25
inches are the most common sizes today. Smaller disks have less surface area and thus store less data than
their larger counterparts; however, they consume less power, can spin faster, and have smaller seek
distances. Historically, as storage densities have increased to where 2–3 gigabytes can fit on a single disk,
the next-smaller diameter in the series has become the most cost-effective and hence the preferred storage
device.
