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Tuesday, April 30, 2013

What is hard disk and what is its purpose?

- HDD or Hard Disk Drive is for data storage. 
- It is used for storage and retrieval of the digital information or data that is stored on it. 
- The data is stored or retrieved by means of its discs that rotate rapidly. 

Hard Disk and its Purpose

- These discs are known as the platters and are coated with some sort of magnetic material. 
- The major characteristic as well as benefit of hard disk drives is that they retain the data even when the power supply is switched off. 
- From hard disk, the data can be read in a manner of random access. 
- This means that the storing and retrieval of the individual blocks of the data can be done either sequentially or in any order that the user may like. 
- A hard disk may consist of one or more than one of those rigid platters. 
These rotating discs have magnetic heads that are located on an actuator arm that is continuously moving and reads and writes data on their surfaces. 
IBM was the first to introduce the hard disk in the year of 1956. 
- Hard disk drives are the most dominant and the prominent secondary storage device for the computers since 1960s. 
- Since then, it has been continuously improved. 
- The HDD units are produced by more than 200 companies; among them most prominent developers are Toshiba, Seagate, Western digital etc. 

HDD’s primary characteristics are:
Ø  Capacity and
Ø  Performance
- The former is specified in terms of the unit prefixes. 
- In some systems, the capacity of the hard disk drive might be unavailable to the user since being used by the operating system and the file system and may have a possibility of occurrence of redundancy.
- The latter is specified in terms of the movements of the heads for a file i.e., the average access time in addition to the time taken for moving the file under the head i.e., the average latency and data rate. 

HDDs are available in two most common factors namely:
Ø  3.5 inch for desktop computers
Ø  2.5 inch for laptops

HDDs might be connected to the system by any of the following standard interface cables:
Ø  Serial ATA or SATA cable
Ø  USB cable
Ø  Serial attached SCSI or SAS cable

- In the year of 2012, flash memory emerged as a tough primary competitor for the hard disk drives. 
- These flash memories are some sort of solid state drives or SSDs. 
- However, the HDDs will still continue to dominate the secondary storage for its advantages such as price per unit of storage and recording capacity. 

- But there is a different scenario is the case of portable electronics.
- Here, the flash drives are considered to be more useful then rotating HDDs because here the durability and physical size of the drive has also to be considered more when compared to price and capacity.
HDD uses the magnetic recording technology where the data is recorded by magnetizing a thin film of material that is typically ferromagnetic on a disk. 
The binary data bits are represented by the sequential change in the direction of the magnetization.
- An encoding scheme is used for encoding the user data. 
- An example of such encoding scheme run – length limited encoding. 
It is these schemes that determine how the magnetic transitions would represent the data.

The latest HDD technologies are:
Ø  Shingled write
Ø  CPP/ MGR heads
Ø  Heat assisted magnetic recording
Ø  Bit – patterned recording

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