Understanding Hard Disk internal geometry

Disk Terminology

Track  A concentric ring on a disk that passes under a single stationary disk head as the disk rotates.

Cylinder The set of tracks with the same nominal distance from the axis about which the disk rotates.

Heads A device called a head reads and writes data in a hard drive by manipulating the magnetic medium that composes the surface of an associated disk platter. Naturally, a platter has 2 sides and thus 2 surfaces on which data can be manipulated; usually there are 2 heads per platter, one per sideSector Section of each disk platter. A sector holds 512 bytes. 
Disk controller A chip and its associated circuitry that control the disk drive.

Blocks and clusters 
A data storage area on a disk. A disk block is 512 bytes.The Unix communities employ the term block to refer to a sector or group of sectors. For example, the Linux fdisk utility normally displays partition table information using 1024-byteblocks, but also uses the word sector to help describe a disk’s size in the phrase, 63 sectors per track.

Clusters are allocation units for data on various file systems (FAT, NTFS, etc.), where data mainly consists of files. Clusters are not directly affected by the physical or virtual geometry of the disk, i.e., a cluster can begin at a sector near the end of a given CH track, and end in a sector on the physically or logically next CH track.
Below images will explain all about disk’s geometry and its internals.

hard disc internals, understanding hard disc geometry


Figure 1. Disk structures:
(A) Track
(B) Geometrical sector
(C) Track sector
(D) Cluster

Below a more detailed explanation is mentioned

  • Mechanical components of a hard disk shown below. 
  • A typical consumer hard disk will usually have between one and five platters.
Hard disc inernals

Design of hard disk has not been much changed since its existence from 1950. The general hard disk design is quite simple, consisting of only a few moving parts. In the picture above you can see:
Platters: Solid disks with a magnetic coating that contains the data. The platters spin at a constant rate when the hard disk is in operation, typically at 3600, 5200 or 7200 rounds per minute (rpm).
Arms: The head stack assembly holds the arms that hold the read/write heads. The stack is rotated by an actuator which is not displayed in the image, causing the arms to position the heads between the hub and the edge of the platter. To achieve great speed and accuracy, the arm and its movement mechanism need to be extremely light and fast. The arm on a typical hard-disk drive can move from hub to edge and back up to 50 times per second.
  • The data is stored on the surface of a platter in areas called tracks and sectors. A sector is displayed in yellow and a track in blue.
hard disc layers

Every platter contains many concentric circles – called tracks – that are used to store the data. This radically differs from a CD or DVD, where a single track of data is used, laid out as a spiral. A modern hard disk has tens of thousands of tracks on a platter. The tracks on a hard disk are divided up into smaller segments called sectors. Each sector usually holds 512 bytes of user data, plus as many as a few dozen additional bytes used for internal drive control and for error detection and correction.

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. Because of this arrangement, often the track location of the heads is not referred to as a track number but rather as a cylinder number. What you should take away from this is that they are essentially the same.

  • This image below shows a cylinder. A cylinder is formed by the tracks that are physically located directly above each other.
hard disc tracks cylinders sectors

In the past, hard disks used to work pretty much exactly as described above. Modern hard disks use methods such as zoned bit recording to improve performance, but these details are only known by the disk controller.

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