Important Points Regarding Seek Time
- It is the time required by read/write head to move from one track to other.
- Most disk scheduling only use seek time.
- It can be reduced if subsequent request belongs to same track or near.
Seek Time = (Time to cross 1 cylinder(track))*(No of cylinder(track) crossed)
Example
Lets look into FCFS Disk scheduling algorithm to learn more about seek time
FCFS (First Come First Serve) is the simplest of all Disk Scheduling Algorithms. In FCFS, the requests are addressed in the order they arrive in the disk queue.
Suppose the order of request is- (82,170,43,140,24,16,190)
And current position of Read/Write head is: 50
So, total overhead movement (total distance covered by the disk arm) =
(82-50)+(170-82)+(170-43)+(140-43)+(140-24)+(24-16)+(190-16) =642
Lets say time to cross 1 track is 1 second
Seek Time = 1 second * 642 track movements
Seek time= 642 s
Seek Time in OS
Seek Time is one of the key components of Disk Scheduling, Before going to Seek Time, let’s first discuss Disk Scheduling. 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. 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.
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