Disk drives map logical blocks of data to physical sectors sequentially, beginning with sector 0 on the outermost track and proceeding inward. Sectors are arranged in cylinders from outer to inner, and tracks within cylinders are mapped in order. Defective sectors are hidden by substituting spare sectors elsewhere. The number of sectors per track and bit density can vary depending on whether constant linear velocity (uniform bit density) or constant angular velocity (uniform rotation speed with decreasing density outward) is used.

1. Disk Structure

2. Disk Structure
• Disk drives are addressed as large 1-
dimensional arrays of logical blocks, where
the logical block is the smallest unit of
transfer
• The 1-dimensional array of logical blocks is
mapped into the sectors of the disk
sequentially

3. Disk Structure Con..
– Sector 0 is the first sector of the first track on
the outermost cylinder
– Mapping proceeds in order through that track,
then the rest of the tracks in that cylinder, and
then through the rest of the cylinders from
outermost to innermost
• Most disk have some defective sectors, but
the mapping hides this by substituting spare
sectors from elsewhere on the disk.

4. Disk Structure Con…
• The number sectors per track is not a
constant on same drives.
• On media that use Constant Linear
Velocity (CLV), the density of bits per track
is uniform.
• The disk rotation speed can stay constant,
and the density of bits decrease from
inner tracks to outer tracks to keep the
rate constant. This Method is used in hard
disk and is known as Constant angular
velocity (CAV)