1. HDD Geometry and Low Level Data Structures
ENE692
LECTURE 6
HDD Geometry and Low Level Data Structures

2. Track, Cylinders, and Sectors
A hard disk is usually made up of multiple platters, each of which use two heads to record and read data, one for the top of the platter and one for the bottom.
The heads that access the platters are locked together on an assembly of head arms. This means that all the heads move in and out together, so each head is always physically located at the same track number. It is not possible to have one head at track 0 and another at track 1,000.
Often the track location of the heads is not referred to as a track number but rather as a cylinder number. A cylinder is basically the set of all tracks that all the heads are currently located at. So if a disk had four platters, it would (normally) have eight heads, and cylinder number 720 (for example) would be made up of the set of eight tracks, one per platter surface, at track number 720.
For most practical purposes, there really isn't much difference between tracks and cylinders--its basically a different way of thinking about the same thing. The addressing of individual sectors of the disk is traditionally done by referring to cylinders, heads and sectors (CHS).

3. Cylinder configuration
Image � Quantum Corporation
This diagram illustrates what "cylinder" means on on a hard disk. This conceptual hard disk spindle has four platters, and each platter has three tracks shown on it. The cylinder indicated would be made up of the 8 tracks (2 per surface) intersected by the dotted vertical line shown.

4. Track density and Areal density
The track density of a hard disk refersto how tightly packed the tracks are on the surface of each platter. Every platter has the same track density. The greater the track density of a disk, the more information that can be placed on the hard disk.
Track density is one component of areal density, which refers to the number of bits that can be packed into each unit of area on the surface of the disk. More is better--both in terms of capacity and performance.
The earliest PC hard disks had only a few hundred tracks on them, and used larger 5.25" form factor platters, resulting in a track density of only a few hundred tracks per inch. Modern hard disks have tens of thousands of tracks and can have a density of 30,000 tracks per inch or more.
The chief obstacle to increasing track density is making sure that the tracks don't get close enough together that reading one track causes the heads to pick up data from adjacent tracks.
To avoid this problem, magnetic fields are made weaker to prevent interference, which leads to other design impacts, such as the requirement for better read/write head technologies and/or the use of PRML methods to improve signal detection and processing.

5. Areal density (1)
Areal density, also sometimes called bit density, refers to the amount of data that can be stored in a given amount of hard disk platter. Since disk platters surfaces are of course two-dimensional, areal density is a measure of the number of bits that can be stored in a unit of area. It is usually expressed in bits per square inch (BPSI).
Being a two-dimensional measure, areal density is computed as the product of two other one-dimensional density measures:
Track Density: This is a measure of how tightly the concentric tracks on the disk are packed: how many tracks can be placed down in inch of radius on the platters. The unit is track per inch (TPI).
Linear or Recording Density: This is a measure of how tightly the bits are packed within a length of track. If in a given inch of a track we can record 200,000 bits of information, then the linear density for that track is 200,000 bits per inch per track (BPI).
To avoid this problem, magnetic fields are made weaker to prevent interference, which leads to other design impacts, such as the requirement for better read/write head technologies and/or the use of PRML methods to improve signal detection and processing.
For example, if we have a platter that is 3.74" in diameter, that's about 1.87 inches. Of course the inner portion of the platter is where the spindle is, and the very outside of the platter can't be used either.  Let's say about 1.2 inches of length along the radius is usable for storage. If in that amount of space the hard disk has 22,000 tracks, the track density of the drive would be approximately 18,333 tracks per inch (TPI).
Every track on the surface of a platter is a different length (because they are concentric circles), and not every track is written with the same density. Manufacturers usually quote the maximum linear density used on each drive.

6. Areal density (2)
Taking the product of these two values yields the drive's areal density, measured in bits per square inch. If the maximum linear density of the drive above is 300,000 bits per inch of track, its maximum areal density would be 5,500,000,000 bits per square inch, or in more convenient notation, 5.5 Gbits/in2. The newest drives have areal densities exceeding 10 Gbits/in2, and in the lab IBM in 1999 reached 35.3 Gbits/in2--524,000 BPI linear density, and 67,300 TPI track density! In contrast, the first PC hard disk had an areal density of about Gbits/in2!
To avoid this problem, magnetic fields are made weaker to prevent interference, which leads to other design impacts, such as the requirement for better read/write head technologies and/or the use of PRML methods to improve signal detection and processing.
For example, if we have a platter that is 3.74" in diameter, that's about 1.87 inches. Of course the inner portion of the platter is where the spindle is, and the very outside of the platter can't be used either.  Let's say about 1.2 inches of length along the radius is usable for storage. If in that amount of space the hard disk has 22,000 tracks, the track density of the drive would be approximately 18,333 tracks per inch (TPI).
Every track on the surface of a platter is a different length (because they are concentric circles), and not every track is written with the same density. Manufacturers usually quote the maximum linear density used on each drive.