Hardware Reference
In-Depth Information
Figure 1.8: Tracks and Cylinders
Each track is further divided into sectors, using some special magnetic
patterns written on the disks at the time of manufacturing. These special
patterns are known as servo sectors (Figure 1.9) which divide a track into equal
segments, and data is written in these segments. Number of servo sectors per
track is the same on all surfaces in an HDD. There are typically 100-200 servo
sectors in any HDD produced these days.
Figure 1.9: Tracks and servo sectors on a disk.
Tracks and sectors are identi fi ed using special magnetic patterns written
on the disks during the production of HDD. In the earlier generation drives
with four or more disks, entire surface of one disk used to be dedicated for
recording these special patterns known as the servo pattern.Sincea lheads
are moved simultaneously by a single actuator, it can be assumed that when the
head on the servo surface (surface containing the servo patterns) is positioned
on the N th track, all other heads are also positioned on the N th track of
their respective surfaces. This assumption started to fall apart with increasing
track density. Slight difference between the thermal properties of arms on the
servo surface and a data surface gives rise to different expansion of two arms
causing an offset between the positions of those two heads. This offset can
no longer be neglected in high track-density drives. Moreover, the scheme
of dedicated servo surface is not suitable for drives with few disks. Earlier
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