Difference between Seek Time and Rotational Latency in Disk Scheduling

Disk management is essential for data storage, retrieval of data and backup the data. To store the data in the disk some I/O operations like read/write are needed. The operating system uses disk scheduling techniques which can be used to schedule I/O requests arriving to the disk. This disk scheduling techniques uses some measurements to improve the performance of read or write operations on the disk. They are disk access time, disk response time, seek time, rotational latency and transfer time. This article discusses about Seek Time and Rotational Latency and its difference.

What is 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. The formula for calculating seek time is

Seek Time = (Time to cross 1 cylinder(track))*(No of cylinder(track) crossed).

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.

What is 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. The formula for calculating rotational latency is

Rotational Latency = (Angle between current position and the required sector) / (Rotational frequency).

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.

Advantage of Seek Time and Rotational Latency

1. These are essential measurements for choosing the best disk scheduling techniques.
2. When both the seek time and rotational latency are reduced that would lead to fast retrieval of data

Difference between Seek Time and Rotational Latency

Conclusion

The Seek Time and the Rotational Latency are the measurements of time which are needed for read/write operations performed on the disk. Both seek time and rotational latency are reduced that would lead to fast retrieval of data. These are essential for choosing the best disk scheduling techniques but seek time is calculated by moving head from one track to another, whereas rotational latency is calculated by moving head from one sector to another.