Data Structures and Algorithm Analysis - Computer Science at ...

Sec. 8.2 Disk Drives 269Boom(arm)PlattersSpindle(a)Read/WriteHeads(b)TrackFigure 8.2 (a) A typical disk drive arranged as a stack of platters. (b) One trackon a disk drive platter.to each head. The data on a single platter that are accessible to any one position ofthe head for that platter are collectively called a track, that is, all data on a platterthat are a fixed distance from the spindle, as shown in Figure 8.2(b). The collectionof all tracks that are a fixed distance from the spindle is called a cylinder. Thus, acylinder is all of the data that can be read when the arms are in a particular position.Each track is subdivided into sectors. Between each sector there are intersectorgaps in which no data are stored. These gaps allow the read head to recognizethe end of a sector. Note that each sector contains the same amount of data.Because the outer tracks have greater length, they contain fewer bits per inch thando the inner tracks. Thus, about half of the potential storage space is wasted, becauseonly the innermost tracks are stored at the highest possible data density. Thisarrangement is illustrated by Figure 8.3a. Disk drives today actually group tracksinto “zones” such that the tracks in the innermost zone adjust their data densitygoing out to maintain the same radial data density, then the tracks of the next zonereset the data density to make better use of their storage ability, and so on. Thisarrangement is shown in Figure 8.3b.In contrast to the physical layout of a hard disk, a CD-ROM consists of a singlespiral track. Bits of information along the track are equally spaced, so the informationdensity is the same at both the outer and inner portions of the track. To keepthe information flow at a constant rate along the spiral, the drive must speed up therate of disk spin as the I/O head moves toward the center of the disk. This makesfor a more complicated and slower mechanism.Three separate steps take place when reading a particular byte or series of bytesof data from a hard disk. First, the I/O head moves so that it is positioned over thetrack containing the data. This movement is called a seek. Second, the sectorcontaining the data rotates to come under the head. When in use the disk is alwayscompromises change over time. In addition, most of the description given here for disk drives is asimplified version of the reality. But this is a useful working model to understand what is going on.