Zoned-bit recording (ZBR) is a method of physically optimizing the utilization of a hard drive by placing more sectors in the outer tracks than in the inner tracks. This technique is also known as zone-bit recording, zone recording, zone-density recording, or multiple-zone recording.
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All hard drives consist of several disks called platters. In each platter, the data is physically placed in concentric circles called tracks. Each track consists of several sectors. The maximum possible number of bits per sector is a constant. As the distance from the center of the platter increases, the circumference of the tracks increases in direct proportion. In early hard drives, all tracks had the same number of sectors. All tracks were arcs with identical measure in angular degrees. Thus, sectors near the edge of the platter were physically longer than those near the center, and the magnetized regions representing data bits were spaced farther apart near the edge of the platter than near the center. As a result, the medium near the outside of each platter was underutilized.
In order to equalize the physical separation between magnetized regions representing bits, sectors should all have the same linear measure, not the same angular measure. In the ideal arrangement, the number of bits, and therefore the number of sectors, per track should vary in direct proportion to the track radius. Zoned-bit recording approaches this ideal by grouping the tracks into sets called zones. Tracks in the inner zones contain the fewest sectors, and tracks in the outer zones contain the most sectors. In this way, the magnetic medium of each platter is utilized as effectively near the outside as near the inside.