This prevents wear on the surface of the disk a

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Unformatted text preview: s of disk systems is to increase the data density of each square inch of disk surface. Access Mechanism Data are recorded on the tracks of a spinning disk surface and read from the surface by one or more read/write heads. As shown in Figure 8.17, the read/write heads are mounted on an access arms assembly. Most disk drives have a single read/write head for each disk surface. But some faster disk systems use multiple heads on each access arm to service a number of adjacent tracks simultaneously. The access arms assembly can be moved in and out in the direction shown in the figure so that the read/write heads can be moved horizontally across the surfaces of the disks. In this manner, the read/write heads can be positioned on any track on which data are to be recorded or from which data are to be read. In case of a disk pack, each usable surface has its own read/write head and all the heads move together. Hence information stored on the tracks, which constitute a cylindrical shape through the disk pack, is accessed simultaneously. Recall the cylindrical storage arrangement of information in a disk pack. The read/write heads are of flying type. That is, they do not have direct contact with the disk surfaces. This prevents wear on the surface of the disk. A separation of about 0.00002 inch is maintained between a read/write head and its corresponding disk surface. In fact, the read/write heads fly so close to the disk surface that if a dust particle (typically of 0.0015 inch size), smoke particle (typically of 0.00025 inch size), finger print (typically of 0.00062 inch size), or a human hair (typically of 0.003 inch size) were placed on the disk surface, it would bridge the gap between the read/write head and the disk surface, causing the head to crash. A head crash, in which the head touches the disk, destroys the data stored in the area of the crash and can destroy a read/write head as well. Access Time Disk access time is the interval between the instant a computer makes a request for transfer of data from a disk system to the primary storage and...
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This document was uploaded on 04/07/2014.

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