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2.1 Disk Structure and Partitions

2.1.2 Disk tracks, cylinders, and sectors

A disk is divided into tracks, cylinders, and sectors. A track is that portion of a disk which passes under a single stationary head during a disk rotation, a ring 1 bit wide. A cylinder is comprised of the set of tracks described by all the heads (on separate platters) at a single seek position. Each cylinder is equidistant from the center of the disk. A track is divided into segments of sectors, which is the basic unit of storage.

On Sun systems a sector is 512 bytes (1 disk block) of data, with header and trailer information. The latter make it possible for the controller to identify sectors, detect data errors, and perform error corrections when necessary. The actual layout of a disk sector will vary depending on the controller, but should look something like that shown in Fig. 2.3. There are two Preambles and a Postamble (whose sizes may vary due to rotational speed, etc., and are disk dependent). The Header field lets the controller know where the head is positioned, and the ECC field is for error correction.

FIGURE 2.3 Sector

The number of sectors per track varies with the radius of the track on the platter. The outermost tracks is larger and can hold more sectors than the inner ones. These outer tracks also spin faster under the head than do the inner ones, because while the angular speed remains the same, the larger circumference results in more sectors spinning by in the same period for the outer tracks. Disk blocks are numbered starting at the outermost track, so put the data you expect to access most often on partition, or slice, 0.

Unix System Administration - 8 AUG 1996
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