Windows 2000 Performance Guide

Disk Architecture

Figure 8-3 illustrates the architecture of a hard disk, which consists of a number of circular platters that rotate around a spindle. The number of platters can vary from one to about a dozen. Data is encoded and stored on both sides of each disk platter. Bits are stored on the platter magnetically. Originally, the presence of a magnetic field in a specific area of the disk represented a single bit of information with a value of 1, and the absence of a magnetic bit in the same area would be interpreted as a 0. Current technology detects the mere change in resistance caused by the presence of a magnetic field in one area, allowing manufacturers to cram more and more data on a magnetic disk. Recording densities in excess of an astonishing 2 billion bits per square inch can be achieved by using this technology—a number that will no doubt already be obsolete by the time you read these words.

Figure 8-3. Hard disk architecture

Information is stored and retrieved from the platters using the recording and playback heads that are attached at the end of each of the arms. The smallest unit of data transferred to and from the disk is a sector. Almost always, the capacity of a sector is 512 bytes. All the sectors on a single platter at a fixed distance from the center of the platter form a track. Generally, data on the disk is addressed using a relative sector number, ...