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I/O System
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  • Introduction to I/O System
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Secondary storage

Secondary storage devices are those devices whose memory is non volatile, meaning, the stored data will be intact even if the system is turned off. Here are a few things worth noting about secondary storage.

  • Secondary storage is also called auxiliary storage.
  • Secondary storage is less expensive when compared to primary memory like RAMs.
  • The speed of the secondary storage is also lesser than that of primary storage.
  • Hence, the data which is less frequently accessed is kept in the secondary storage.
  • A few examples are magnetic disks, magnetic tapes, removable thumb drives etc.

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Magnetic Disk Structure | Structure of a magnetic disk

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.

Magnetic Disk

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.

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Disk scheduling properties | Seek Time | Rotational Latency | Transfer time

Disk scheduling properties:

There are some of the important terms:

  • Seek Time:Seek time is the time taken to locate the disk arm to a specified track where the data is to be read or write. So the disk scheduling algorithm that gives minimum average seek time is better.
  • Rotational Latency: Rotational Latency is the time taken by the desired sector of disk to rotate into a position so that it can access the read/write heads. So the disk scheduling algorithm that gives minimum rotational latency is better.
  • Transfer Time: Transfer time is the time to transfer the data. It depends on the rotating speed of the disk and number of bytes to be transferred.

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