Rotational Latency

The time required by the read/write head to move from the current sector to the desired sector is called the rotational latency. The rotational latency can be calculated by dividing the angle between the current sector and the desired sector by the rotational frequency.

The formula for calculating the rotational latency is given by:

Rotational latency = (angle between current sector and desired sector) / Rotational frequency

Average Rotational Latency

The half of the time required for one revolution is called average rotational latency. Average rotational latency can be defined by the ratio of time for 1 rotation divided by 2.

Average Rotational Latency = Time for 1 revolution / 2

How to Find Average Rotational Latency?

Below are the steps to find average rotational latency:

• Find the RPM (Rotation Per Minute) given in the question.
• Then, find the time required for 1 rotation.
• After finding the 1 rotation time, use the formula: Average Rotational Latency = Time for 1 revolution / 2

Important Points on Rotational Latency

• Rotational latency can be minimized if the sectors are adjacent.
• Rotational latency depends on the disk’s rotation speed given in RPM i.e., Rotation Per Minute.
• Rotational latency is also called as the Rotational Delay.
• The delay for the rotation of disk to bring the desired sector under read/write head is called rotational latency.
• The Rotational latency depends on the speed of the spindle of the disk.
• The number of sectors in one track can be covered in one rotation of the disk.

Solved Examples on Rotational Latency

Example: Calculate the average rotational latency if disk rotation is 6000 RPM.

Solution:

6000 rotations in 60 sec

1 rotation = 60 / 6000 sec

1 rotation = 10 msec

Average rotational latency = Time for 1 rotation / 2

Average rotational latency = 10 msec / 2

Average rotational latency = 5 msec

Example: Calculate the average rotational latency if disk rotation is 3000 RPM.

Solution:

3000 rotations in 60 sec

1 rotation = 60 / 3000 sec

1 rotation = 20 msec

Average rotational latency = Time for 1 rotation / 2

Average rotational latency = 20 msec / 2

Average rotational latency = 10 msec

Example: Calculate the average rotational latency if disk rotation is 1200 RPM.

Solution:

1200 rotations in 60 sec

1 rotation = 60 / 1200 sec

1 rotation = 50 msec

Average rotational latency = Time for 1 rotation / 2

Average rotational latency = 50 msec / 2

Average rotational latency = 25 msec

Example: Calculate the average rotational latency if disk rotation is 2400 RPM.

Solution:

2400 rotations in 60 sec

1 rotation = 60 / 2400 sec

1 rotation = 25 msec

Average rotational latency = Time for 1 rotation / 2

Average rotational latency = 25 msec / 2

Average rotational latency = 12.5 msec

Example: Find the rotational latency of the disk if the angle between current sector and desired sector is 60 degrees and rotational frequency is 10 MHz.

Solution:

Angle between current sector and desired sector = 60°

Rotational frequency = 10 MHz

Rotational latency = (angle between current sector and desired sector) / Rotational frequency

Rotational latency = 60 / 10 MHz

Rotational latency = 6 microseconds

Example: Find the rotational frequency of the disk, given the angle between the current and the desired sector is 30 and rotational latency is 10 ns.

Solution:

Angle between current sector and desired sector = 30°

Rotational Latency = 10 ns

Rotational latency = (angle between current sector and desired sector) / Rotational frequency

Rotational frequency = 30 / 10 ns

Rotational frequency = 3 GHz

Frequently Asked Questions on Rotational Latency – FAQs

What is Rotational Latency of Disk?

The time required by the read/write head to reach the desired sector from the current sector is called as the rotational latency of the disk.

Write the Formula for the Average Rotational Latency.

The average rotational latency is given by:

Average Rotational Latency = Time required for 1 rotation / 2