Disk scheduling is done by operating systems to schedule I/O requests arriving for the disk. Disk
scheduling is also known as I/O scheduling.
Disk scheduling is important because:
Multiple I/O requests may arrive by different processes and only one I/O request can be served at a time
by the disk controller. Thus other I/O
requests need to wait in the waiting queue and need to be scheduled.
Two or more request may be far from each other so can result in greater disk arm movement.
Hard drives are one of the slowest parts of the computer system and thus need to be accessed in an
efficient manner.
There are many Disk Scheduling Algorithms.
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Seek Time:Seek time is the time taken to locate the disk arm to a specified track
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.
Disk Access Time:
Disk Access Time = Seek Time + Rotational Latency + Transfer Time
Disk Response Time: Response Time is the average of time spent by a request waiting to perform its
I/O operation. Average Response time is the response time of the all requests. Variance Response Time is
measure of how individual request are serviced with respect to average response time. So the disk scheduling
algorithm that gives minimum variance response time is better.
FCFS is the simplest of all the Disk Scheduling Algorithms. In FCFS, the requests are addressed in the
order they arrive in the disk queue.
Advantages of FCFS :
1.There is no indefinite delay.
2.There is no starvation ,because each request gets a fair chance.
Disadvantages of FCFS :
1.FCFS scheduling is not offered as the best service.
2.Scheduling disk time is not optimized.
In SSTF (Shortest Seek Time First), requests having shortest seek time are executed first. So, the seek
time of every request is calculated in advance in the queue and then they are scheduled according to
their calculated seek time.
Advantages of SSTF :
1.The average response time is decreased.
2.Increased throughput.
Disadvantages of SSTF :
1.There may be a chance of starvation.
2.SSTF is not an optimal algorithm.
In SCAN algorithm the disk arm moves into a particular direction and services the requests coming in its
path and after reaching the end of disk, it reverses its direction and again services the request
arriving in its path. So, this algorithm works as an elevator and hence also known aselevator
algorithm.
Advantages of SCAN :
1.There is a low variance of response time.
2.Throughput is high.
3.Response time is average and No starvation.
Disadvantages of SCAN :
1.Long waiting time.
2.We have to move the disk head to the end of the disk even when we don’t have any request to service.
In SCAN algorithm, the disk arm again scans the path that has been scanned, after reversing its
direction. So, it may be possible that too many requests are waiting at the other end or there may be
zero or few requests pending at the scanned area.
Advantages of CSCAN :
1.C-SCAN offers better uniform waiting time.
2.It offers a better response time.
Disadvantages of CSCAN :
1.There are more seek movements as compared to SCAN disk scheduling.
2.We have to move the disk head to the end of the disk even when we don’t have any request to
service.
It is similar to the SCAN disk scheduling algorithm except for the difference that the disk arm in spite
of going to the end of the disk goes only to the last request to be serviced in front of the head and
then reverses its direction from there only.
Advantages of LOOK :
1.There is no starvation.
2.It offers low variance in waiting time and response time.
Disadvantages of LOOK :
1.There is more overhead to find the end request.
2.Look disk scheduling is not used in case of more load.
As LOOK is similar to SCAN algorithm, in similar way, CLOOK is similar to CSCAN disk scheduling
algorithm. In CLOOK, the disk arm in spite of going to the end goes only to the last request to be
serviced in front of the head and then from there goes to the other end’s last request.
Advantages of CLOOK :
1.There is no starvation.
2.The performance of the C-Look scheduling is better than Look disk scheduling.
3.It offers low variance in waiting time and response time.
Disadvantages of CLOOK :
1.There is more overhead in calculations.