Disk
Structure
The traditional
head-sector-cylinder, HSC numbers are mapped to linear block addresses by
numbering the first sector on the first head on the outermost track as sector
0. Numbering proceeds with the rest of the sectors on that same track, and then
the rest of the tracks on the same cylinder before proceeding through the rest
of the cylinders to the center of the disk.
In modern practice linear
block addresses are used in place of the HSC numbers for a variety of reasons:
1.
The linear length of tracks near the
outer edge of the disk is much longer than for those tracks located near the
center, and therefore it is possible to squeeze many more sectors onto outer
tracks than onto inner ones.
2.
All disks have some bad sectors, and
therefore disks maintain a few spare sectors that can be used in place of the
bad ones. The mapping of spare sectors to bad sectors in managed internally to
the disk controller.
3.
Modern hard drives can have thousands of
cylinders, and hundreds of sectors per track on their outermost tracks. These
numbers exceed the range of HSC numbers for many (older ) operating systems,
and therefore disks can be configured for any convenient combination of HSC
values that falls within the total number of sectors physically on the drive.
Modern disks pack many
more sectors into outer cylinders than inner ones, using one of two approaches:
·
With Constant Linear
Velocity, CLV, the density of bits is uniform from cylinder to
cylinder. Because there are more sectors in outer cylinders, the disk spins
slower when reading those cylinders, causing the rate of bits passing under the
read-write head to remain constant. This is the approach used by modern CDs and
DVDs.
·
With Constant Angular
Velocity, CAV, the disk rotates at a constant angular speed, with
the bit density decreasing on outer cylinders. ( These disks would have a
constant number of sectors per track on all cylinders. )
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