Magnetic Disk Structure
In modern computers, most of the secondary storage is in the form of magnetic disks. Hence, knowing the structure of a magnetic disk is necessary to understand how the data in the disk is accessed by the computer.
Structure of a magnetic disk
A magnetic disk contains several platters. Each platter is divided into circular shaped tracks. The length of the tracks near the centre is less than the length of the tracks farther from the centre. Each track is further divided into sectors, as shown in the figure.
Tracks of the same distance from centre form a cylinder. A read-write head is used to read data from a sector of the magnetic disk.
The speed of the disk is measured as two parts:
- Transfer rate: This is the rate at which the data moves from disk to the computer.
- Random access time: It is the sum of the seek time and rotational latency.
Seek time is the time taken by the arm to move to the required track. Rotational latency is defined as the time taken by the arm to reach the required sector in the track.
Even though the disk is arranged as sectors and tracks physically, the data is logically arranged and addressed as an array of blocks of fixed size. The size of a block can be 512 or 1024 bytes. Each logical block is mapped with a sector on the disk, sequentially. In this way, each sector in the disk will have a logical address.