Geometry of Hard Disk Drive


Overview of Hard Disk Drive Geometry

The geometry of a hard disk drive is the organization of data on platters. The geometry of the hard disk specifies how and where the data is stored on the surface of each platter the geometry of the hard disk drive is usually specified by the following numerical values.






The head of a hard disk drive represents the total number of sides on all the platters that store data. For example if d disk drive has 8 platters, the hard disk drive can have maximum up to 16 heads. Some hard drive manufacturers use a technology called Sector translation. As per this technology a hard disk drive can have more than two heads per Platte. The before ad per sector transition, it is possible that a hard disk drive can haves 12 heads on only one platter. However, irrespective of technology used for manufacturing hard disk drive, the maximum number of heads a hard disk drive can contain is 16.

All the data on a hard disk drive is stored on concentric circles on the surface of each head. Each concentric circle is called track. All tracks are numbered, starting from zero, starting at the outside of the platter and increasing as you go in. A set of all tracks of same diameter present on a head is called a cylinder. It is the number of cylinder that is used for measuring the drive geometry and the number of tracks.
The number of cylinders in a drive and the number of tracks on platter in drive are exactly same. Both these number are determined by the manufacturing the drive. In most hard disks, the number of cylinders is set by a magnetic pattern called a servo pattern.

Sector per Tracks:-
A sector is the basic unit of data storage on a hard disk. The term “sector” emanates from a mathematical term referring to that pie shaped angular section of a circle, bounded on two sides by radii and the third by the perimeter of the circle. An explanation in its simplest form, a hard disk is comprised of a group of predefined sectors that form a circle. That circle of predefined sectors is defined as a single track.
A group of concentric circles (tracks) define a single surface of a disks platter. Early hard disks had just a single one-sided platter, while today’s hard disks are comprised of several platters with tracks on both sides, all of which comprise the entire hard disk capacity.

Write Precompensation:-
As discussed in the section on zoned bit recording, older hard disks used the same number of sectors per track. This meant that older disks had a varying bit density as you moved from the outside edge to the inner part of the platter. Many of these older disks required that an adjustment be made when writing the inside tracks and a setting was placed in the BIOS to allow the user to specify at what track number this compensation was to begin.
This entire matter is no longer relevant to modern hard disks, but the BIOS setting remains for compatibility reasons. Write precompensations not done with today’s drives; even if it were, the function would be implemented within the integrated controller and would be transparent to the user.

Landing zone:-
The earlier hard disks are highly prone to damage when the machine is switched off. As soon as the computer is switched off, the platters of the hard disk stop spinning and the airflow that keeps the heads flying stops. This head will then land on the disk drive, however there is a possibility that the head lands on a cylinder that already contains data. This may result in disk damage. To avoid this situation an unused cylinder number is specified in the BIOS setting where the heads will land when the machine is switched off. The value is called the landing zone.
However, like write precompensation, this is also not applicable for the modern hard disks. This is because the modern hard disk has a mechanism to automatically write the heads o a special area on the hard disk when the computer is switched off.


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