Cluster sizes vary depending on the size and type of

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Unformatted text preview: number. Before a disk drive can access a piece of data (a record) stored on a disk, it must specify the record's disk address. The disk address is comprised of the sector number, track number, and surface number (when double-sided disks are used). That is, the disk address represents the physical location of the record on the disk.170 We saw that disk drives are designed to access only whole sectors at a time (a sector being the smallest unit of data access from a disk). Since a file usually is not of a size that is an even multiple of sector size (usually 512 bytes), some part of the last sector allocated to the file may be unused. Thus, on an average half of the sector size is wasted for each file stored on the disk. Hence it is recommended that the sector size for a disk should not be too large. The sector size should not even be too small, otherwise the size of the mapping table used to map disk addresses to data records will become too large. The DOS operating system goes one step further of combining two or more sectors to form a cluster. In this case, the smallest unit of data access from a disk becomes a cluster -not a sector. That is, read/write operations read/write a whole cluster at a time. Cluster sizes vary, depending on the size and type of the disk, but they can range from 2 to 64 sectors per cluster. Notice that a cluster-based disk organization may lead to more wastage of disk space as compared to sector-based organization, because in case of cluster-based systems, on an average, half of the cluster size is wasted for each file stored on the disk. However, cluster-based organization, leads to less management overhead for mapping of data records to physical disk locations. Often multiple disks are stacked and used together to create large capacity disk storage systems. In this case, a set of magnetic disks is fixed to a central shaft one below the other to form a disk pack (see Figure 8.16). The disk pack is sealed and mounted on a disk drive, which consists of a motor to rotate the disk pack about its axis. The disk drive also has an access arm assembly that has separate read/write heads for each surface of the disk...
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This document was uploaded on 04/07/2014.

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