Difference between Seek Time and Rotational Latency in Disk Scheduling

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 Time Rotational Latency
1 It 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.
2 Most 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.
3 It can be reduced if subsequent request belongs to same track or near. It can be reduced if subsequent request belongs to adjacent sector.
4 Seek 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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