Magnetic disk is the secondary storage device used to support direct access to a desired location.
The different parts that are present in magnetic disk or hard disk are explained below. All these parts are helpful to read, write and store the data in the hard disk.
Disk blocks − The unit data transfer between disk and main memory is a block. A disk block is a contiguous sequence of bytes.
Track − Blocks are arranged in concentric rings called tracks.
Sectors − A sector is the smallest unit of information that can be read from or written to disk: for example: sector size of 512 bytes.
Platters − The surface of the platter is covered with a magnetic material. Information is recorded on this surface. The set of all tracks with the same diameter is called a cylinder. Typical platter diameters are 3.5 inch or 5.2 inch.
Read-write head − Each platter has a read-write head on both sides. It is used for reading and writing the data on a platter.
Disk controller − A disk controller interfaces a disk driver to the computer.
The time to read or write a block varies depending on the location of data. The performance of a hard disk can be calculated by using the below mentioned formula.
Access time = seek time+ rotational delay + transfer time
Seek time − The time to move the disk-head to the track on which a desired block is located.
Rotational delay − It is the waiting time for the desired block to rotate under disk head.
Transfer time − It is the time to read or write the data in the block once the head is positioned.
The diagram of a magnetic disk or a hard disk is given below −
Let us now understand how a request or data is placed on a hard disk. We will learn about the same with the help of an example.
When multiple I/O request arrive, the operating system has to choose an order, which I/O request has to be served.
The diagram given below explains how a request is placed on a disk −