Difference between Rotational Latency and Disk Access Time in Disk Scheduling

Last Updated : 10 May, 2025

When a computer reads or writes data on a hard disk, it takes a little time to find and access that data. This delay is caused by two main factors: rotational latency and disk access time. Rotational latency is the time it takes for the disk to spin and bring the required part of the disk under the read/write head. Disk access time includes this rotational delay as well as the time needed to move the head to the right track and actually read or write the data.

In disk scheduling, different techniques are used to manage how these delays are handled, so the system can access data faster and work more efficiently.

1. Rotational Latency

Rotational Latency, also known as rotational time, is the amount of time taken by the disk to rotate the track so that the read/write head reaches the exact sector. In other words, it is the time taken for the desired sector to come under the read/write head. Rotational latency depends on the rotational speed of the spindle.

Rotational Latency = \frac{\text{(Angle between current sector and required sector)}}{\text{(Rotational frequency) }}
Average Rotational Latency = \frac12 \times \text{One rotation time}

Each platter has its own read/write head. The head can only move forward and backward. When the head moves forward, it is moving toward the innermost track. When the head moves backward, it is moving toward the outermost track.

Best Case: The head is already positioned on the desired sector.
Worst Case: The head is on a sector that is far from the desired one, requiring a full disk rotation to reach it.
Average Case: Equal to half of the full rotation time.

Example:

Let Speed = 2400 RPM (Rotation Per Minute)
Then, 2400 Rotation = 1 min
1 Rotation = 60 sec / 2400
1 Rotation = 1/40 sec

One Rotation Time = 1/40 sec

Average Rotational Latency = (1/2) * (1/40) = 1/80 sec

2. Disk Access Time

Disk Access Time is the total time taken by the computer to process a data request from the processor and then transfer the data. When a program needs to read information from the disk, the system must rotate the disk to position the correct track and sector where the data resides.

Disk Access Time = Seek time + Rotation time + Rotational Latency + Transfer time

Disk Access Time Includes:

Seek Time:

Seek time is the time taken by the read/write head to reach the desired track.
It is considered the most important component of disk access time, as it directly impacts performance.
Seek time is inversely proportional to performance: the lower the seek time, the better the performance.
Average Seek Time = 1/3 × Time for one full stroke

Rotation Time:

This is the total time taken for one complete rotation of the disk (360 degrees).
To reach the desired sector, the disk rotates using the spindle, either in a clockwise or counter-clockwise direction—but only one direction at a time.

Rotational Latency:

Rotational latency is the time taken for the desired sector to come under the read/write head after the correct track has been located.
Average Rotational Latency = 1/2 × One rotation time

Data Transfer Time:

This is the time required to actually transfer the data after the head is positioned over the correct sector.
Data Transfer Time = Data to be transferred / Transfer rate

Difference between Rotational Latency and Disk Access Time in Disk Scheduling

ROTATIONAL LATENCY	DISK ACCESS TIME
The time taken by the desired sector in disk to come under read/write head is known as Rotational Latency.	Disk Access Time is basically the time required by computer to process a read/write request and also to retrieve required data.
Rotational Latency depends on rotational speed of the spindle.	Disk Access Time depends on two parts i.e. access time and data transfer time.
Average rotational latency can be written as : Avg Rotational Latency = (1/2) * One rotation time.	Disk Access Time can be written as : Access time + Data Transfer Time.
Rotational Latency Time can be reduced if subsequent request belongs to adjacent sector.	We can reduce Disk Access Time if we are able to reduce access time and data transfer time.
Rotational Latency = (Angle between current sector and the required sector) / (Rotational frequency).	Disk Access Time = Seek time + Rotational Latency + Data Transfer Time