MAGNETIC DISK

§  Magnetic Disk can be considered as higher capacity version of Floppy.

§  In place of using one magnetic film of doubled sided, as used in floppy, magnetic disk uses many surfaces of metal plates which are more reliable than magnetic film of floppy.

§  These metal plates help to increase the overall storage capacity of disk.

§  Also, these metal plates help to provide faster access and storage of data than Floppy.

§  Magnetic Disk is also known as Hard Diskbecause it uses hard metal plates for  data storage.

§  The magnetic disk surfaces are also divided into tracks and sectors just like floppy.

§  However, in the magnetic disk, there is one additional structure known as cylinder is used.

§  A Magnetic Disk contains a number of cylinders equal to the number of tracks on a disk’s surface.

§  Each surface of magnetic disk is provided with a separate read/write head for reading and writing data to these surfaces.

§  Magnetic Disk is the most popular type of direct access media.

§  The capacity of Magnetic disk is much larger than the floppy disk.

§  For example: A floppy disk capacity is approximately 1.44 MB, whereas capacity of magnetic disk is around 100s of GB.

§  Also, the performance of hard disk, in terms of speed of data access and storage, and reliability is much higher than Floppy.

§  The Data Access Time of a Magnetic Disk is defined Below:

Access Time:

§  The time required to access data from disk is known as Access Time.

§  This time consists three components:

·         Seek Time

·         Rotational Time

·         Data Transfer Time

Seek Time:

§  This is the time taken by Read/ Write head to move itself from its present Position to Position from which data is to be accessed.

Rotational Time:

§  This is the time taken in rotation of disk to move the position of the data, to be accessed, under the Read/ Write head.

§  An average rotational time or Latency time is equal to time taken in half rotation of disk.

Data Transfer Time:

§  This is the time taken in transfer of accessed data from disk to primary memory.

Advantages of Magnetic Disk:

§  Provide very fast data access because it is direct access storage medium.

§  Provide low cost storage medium.

§  Rewriteable storage medium, help to quickly write the new updated information over area where old information was written.

§  Files on Magnetic Disk can be stored in an interrelated manner, which help to update a common transaction in all required files simultaneously, without re-entry of transaction to individual files.

§  Provide large storage capacity to store large data volume, multiple magnetic disks can also be used together to provide large memory capacity.

§  Used as storage medium for online real – time data processing applications like in banks, etc.

§  Hard Disk is used as online storage device for access and storage of data.

§  Due to its random access property, magnetic disks are often used as online secondary storage devices, in which case they store data of multiple users of the computer system. A tape is not suitable for such type of usage, due to its sequential-access property.

§  Due to their low cost and high data recording densities that magnetic dis, the cost per bit of storage is low for magnetic disks. An additional cost benefit is that magnetic disks can be erased and reused many times.

§  Data transfer rate for a magnetic disk system is normally higher than a tape system.

Disadvantages of Magnetic Disk:

§  When new data is stored on a Magnetic Disk, It is stored with a technique known as overlying; where in new data is stored on old data as a new layer. This type of writing data does not provide audit trail of previous stored data.

§  Magnetic Disk is a fixed type of disk; it cannot be moved easily from one place to another, Therefore, It is difficult to use magnetic disk as a portable memory.

§  It is more difficult to maintain the security of information stored on magnetic disks, which are used as shared, online secondary storage devices as compared to information stored on magnetic tapes or on other types of magnetic disks.

§  On a cost-per-bit basis, the cost of magnetic disks is low, but the cost of magnetic tapes is even lower.

§  Floppy disks, zip disks and disk packs should be labelled properly to prevent erasure of useful data by mistake.

§  They must be stored in a dust-free environment.

Uses of Magnetic Disk:

§  For applications that are based on random data processing.

§  As a shared, online secondary storage device. Winchester disks and disk packs are often used for this purpose.

§  As a backup device for offline storage of data, so that if by accident, the data on an online storage device is corrupted, it can be retrieved from the backup storage. Floppy disks, zip disks, and disk packs are often used for this purpose.

§  Archiving of data which are not used frequently, but which may be used once in a while.

§  Floppy disks, zip disks and disk packs are often used for this purpose.

§  Transferring of data and programs from one computer to another, which is not linked together. Floppy disks and zip disks are often used for this purpose.

§  Distribution of software by vendors. Originally sold software or software updates are often distributed by vendors on floppy disks and zip disks.

 

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