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Difference between Seek Time and Rotational Latency in Disk Scheduling

  • Last Updated : 01 Apr, 2020

Seek Time:
A disk is divided into many circular tracks. Seek Time is defined as the time required by the read/write head to move from one track to another.

Example,
Consider the following diagram, the read/write head is currently on track 1.

Now, on the next read/write request, we may want to read data from Track 4, in this case, our read/write head will move to track 4. The time it will take to reach track 4 is the seek time.



Rotational Latency:
The disk is divided into many circular tracks, and these tracks are further divided into blocks knows as sectors. The time required by the read/write head to rotate to the requested sector from the current position is called Rotational Latency.

Example,
Consider the following diagram, We have divided each track into 4 sectors.
The systems get a request to read a sector from track 1, thus the read/write head will move to track 1 and this time will be seek time.
The read/write head is currently in sector 3.

But the data may not be in sector 3. The data block may be present in sector 1. The time required by read/write head to move from sector 3 to sector 1 is the rotational latency.
Below is the final configuration.

Let’s see the difference between rotational latency and seek time.

S.NO.Seek TimeRotational Latency
1It is the time required by read/write head to move from one track to other.It is the time required by read/write head to move from one sector to other.
2Most disk scheduling only use seek time.Most disk scheduling do not consider rotational frequency because in most modern system, the actual physical location of blocks in not available.
3It can be reduced if subsequent request belongs to same track or near.It can be reduced if subsequent request belongs to adjacent sector.
4Seek Time = (Time to cross 1 cylinder(track))*(No of cylinder(track) crossed).Rotational Latency = (Angle between current position and the required sector) / (Rotational frequency).

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