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Thursday, May 30, 2013

What are the various Desk Scheduling methods?

About Disk Scheduling

The I/O system has got the following layers:
  1. User processes: The functions of this layer including making I/O calls, formatting the I/O and spooling.
  2. Device independent software: Functions are naming, blocking, protection, allocating and buffering.
  3. Device drivers: Functions include setting up the device registers and checking their status.
  4. Interrupt handlers: These perform the function of waking up the I/O drivers up on the completion of the I/O.
  5. Hardware: Performing the I/O operations.
- Disk drives can be pictured as large 1 – D array consisting of logical blocks that are smallest unit of transfer.  
- These blocks are mapped in to the disk sectors in a sequential manner. 
Mapping is done in the same manner. 
- The responsibility of using the hardware efficiently is the duty of the operating system for the disk drives for increasing the speed of access and bandwidth of the disk. 

Algorithms for Scheduling Disk Requests

There are several algorithms existing for the scheduling of the disk requests:

- In this method the request having the minimum seek time is selected from the present head position. 
- This method is a modification of the SJF (shortest job first) scheduling and therefore contains some possibility of process starvation.

- From one end of the disk, the disk arm starts and continues in the direction of the other end, serving to the requests till the opposite end. 
- At this end the head is reversed and the process continues. 
- This is sometimes called as the elevator algorithm.

Ø  C – SCAN: 
- A better algorithm then the previous one. 
- This one offers a more uniform waiting time than the previous one. 
- The movement of the head is from one end to another while it services the requests encountered along the way. 
- However, the difference is that when it comes to the other it straightaway goes to the beginning without heeding to any of the requests in the way and then again starts. 
- The cylinders are treated as the circular list wrapped around last and the first cylinder.

Ø  C – Look: 
- This is a modified version of the C – SCAN. 
- Here the arm or the head travels only up to the last request rather than going till the far end. 
- Then immediately the direction is reversed and the process continues.

- For disk scheduling it is important that the method be selected as per the requirements only. 
- The first one is the most commonly used and appeals to the needs naturally. 
- For a system where often there is a heavy load on the disk, the SCAN and C- SCAN methods can help. 
- The number as well as the kind of requests affects the performance in a number of ways.
- On the other hand, the file – allocation method influences the requests for the disk services. 
- These algorithms have to be written as an individual module of the OS so that if required it can be replaced with a different one easily. 
- As a default algorithm, the LOOK or the SSTF is the most reasonable choice. 

Ways to attach to a disk

There are two ways of attaching the disk:
Ø  Network attached: This attachment is made via a network. This is called the network attached storage. All such connected storage devices together form the storage area network.
Ø  Host attached: This attachment is made via the I/O port.

All these disk scheduling methods are for the optimization of the secondary storage access and for making the whole system efficient. 

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