Faster access of data is achieved by cylinder based

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Unformatted text preview: pack on which data can be recorded. Normally, the upper surface of the top disk and the lower surface of the bottom disk are not used in a disk pack because these surfaces may get easily scratched. The access arm assembly is designed in such a way that all the access arms (on which the read/write heads are fixed) for all the disk surfaces move together. Thus, if the read/write head that serves the 0 th recording surface is positioned over the 5th track, each of the heads on the arms that serve other recording surfaces is similarly positioned over the 5th track. For faster access of data from disk packs, a concept called cylinder is used for data organization on disk packs. As shown in Figure 8.16, a set of corresponding tracks on all the recording surfaces of a disk pack together form a cylinder. For example, the 5th track of all the recording surfaces together comprise the 5th cylinder of the disk pack. Thus if there are 200 tracks on a single disk surface, there are 200 cylinders in the disk pack. Obviously, in this case the disk address of a data record is comprised of sector number, cylinder number, and surface number (track number is not required because track number and cylinder number are same). Faster access of data is achieved by cylinder-based organization by avoiding movement of access arms when large number of related records are to be processed in sequence. The related records of a file can be stored on the same cylinder of the multiple disks of a disk pack so that in one revolution of the disk pack, all records stored on say track 5 of surface 0 are read. In the next revolution, the subsequent records stored on track 5 of surface 1 are read, in the revolution after that, the subsequent records stored on track 5 of surface 2 are read, and so on. This procedure can continue down the cylinder without any delays caused by the movement of access arms. Storage Capacity As you might have already guessed, the storage capacity of a disk system depends on the following: 1. Number of recording s...
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