Hard Disk Storage - Upgrading and Repairing PCs - page 576

Virtually all ATA drives use a technique called zoned-bit recording (ZBR), sometimes
shortened to zoned recording , which writes a variable number of sectors per track.
Withoutzonedrecording,thenumberofsectors(andthereforebits)oneachtrackisacon-
stant.Thismeanstheactualnumberofbitsperinchwillvary.Morebitsperinchwillexist
on the inner tracks, and fewer will exist on the outer. The data rate and rotational speed
will remain constant, as will the number of bits per track. Figure 9.7 shows a drive recor-
ded with the same number of sectors per track.
Figure 9.7 Standard recording, where the same number of sectors comprises every track.
A standard recording wastes capacity on the outer tracks because it is longer yet holds the
same amount of data (more loosely spaced) as the inner tracks. One way to increase the
capacity ofa hard drive during the low-level format is to create more sectors onthe disks'
outercylindersthanontheinnerones.Becausetheyhavealargercircumference,theouter
cylinders can hold more data. Drives without zoned recording store the same amount of
data on every cylinder, even though the tracks of the outer cylinders might be twice as
long as those ofthe inner cylinders. The result is wasted storage capacity because the disk
medium must be capable of storing data reliably at the same density as on the inner cyl-
inders. When the number of sectors per track is fixed, as in older controllers, the drive
capacity is limited by the density of the innermost (shortest) track.
Drives that use zoned recording split the cylinders into groups called zones , with each
successive zone having more sectors per track as you move outward from the center of
the disk. All the cylinders in a particular zone have the same number of sectors per track.
The number of zones varies with specific drives, but most drives have between 16 and 32
zones.
Figure 9.8 shows a drive with zoned-bit recording.
Figure 9.8 Zoned recording, where the number of sectors per track increases within each zone, moving out
from the center.