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What are Magnetic Disks?
A magnetic disk is a storage device that can be assumed as the shape of a Gramophone record. This disk is coated on both sides with a thin film of Magnetic material. This magnetic material has the property that it can store either ‘1’ or ‘0] permanently. The magnetic material has square loop hysteresis (curve) which can remain in one out of two possible directions which correspond to binary ‘1’ or ‘0’.
Bits are saved in the magnetized surface in marks along concentric circles known as tracks. The tracks are frequently divided into areas known as sectors.
In this system, the lowest quantity of data that can be sent is a sector. The subdivision of one disk surface into tracks and sectors is displayed in the figure.
Access time
The access time of a record on a disk includes three components such as seek time, latency time, and data transfer time.
Seek time − The time required to arrange the read/write head at the desired track is called seek time. For example, suppose that the read/write head is on track 2 and the record to be read is on track 5, then the read/write head must move from track 2 to track 5. The average seeks time on a modern disk is 8 to 12 ms.
Rotational delay or latency time − The time required to position the read/write head on a specific sector when the head has already been placed on the desired track is called rotational delay. The rotational delay is based on the speed of rotation of the disk. On average the latency will be half of one revolution time. The average latency time on modern disks is 4.2 to 6.7ms.
Data transfer time − Data transfer time is the actual time needed to send the data.
Advantages of Magnetic Disk
Access time − With a magnetic disk, it is achievable to access a record explicitly. Therefore access time is less in this case.
Flexibility − Magnetic disk has to be the flexibility of being used as a sequential as well as direct access storage device.
Transmission Speed − The rate of data transfer is fast in a magnetic disk.
Reusable − It can remove a specific data and save another data at the same place.
Storage Capacity − It can store a very large amount of data.
Disadvantages of Magnetic Disk
Cost − The cost of per character storage is much higher as compared to magnetic tape.
Non-Portability − Portability of it is very less as compared to magnetic tape.
Limited size record − Duration of record which can be saved on it is limited by the size of disk track or disk sector.
Non-human readable − Data stored on it is not in human-readable form, therefore manual encoding is not possible at all.
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