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.
Blocks and clusters
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.
Figure 1. Disk structures:
(B) Geometrical sector
(C) Track sector
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.
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.
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.
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.