# Difference between Rotational Latency and Disk Access Time in Disk Scheduling

1. Rotational Latency :
Rotational Latency is also known as Rotational time. The amount of time taken by the disk to rotate the track when the read/write head come to exact sector or we can also say that the time taken by the desired sector to come under the read/write head is called rotational latency. Rotational Latency is depends on the rotational speed of the spindle.

```Rotational Latency,
= (Angle between current sector and required sector) / (Rotational frequency) ```

And,
Average Rotational Latency,
= (1/2) * One rotation time

All the platter has its own read/write head. The head can only be move forward and backward. If the header is move forward it means that the header is moving towards innermost track. If the header is move backward it means that the header is moving towards outermost track.

• Best Case, = When the header is already in the desired sector.
• Worst Case = When the header is at the sector which is far away from the desired sector. You have to wait for one complete rotation.
• Average Case = Half of the rotation time.

Example –

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

One Rotation Time,
= 1/40 sec

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

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 has to read information from the disk, the system should rotate the circle, the track and the sector on which the data is resides.

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

Disk Access Time includes –

1. Seek Time –
It is the time taken by the read/write head to reach to the desired output. It is known to be the most important time because it cannot create a gap. Seek time is inversely proportional to the performance. Lesser the seek time, the more good will be the performance.
```Average seek time,
= (1/3) * time taken for one full stroke ```
2. Rotation Time –
It is the total time taken for one complete rotation (360 degree). For reaching to the desired sector, the disk will rotate by spindle in either clockwise direction or anti-clockwise direction, but can be move in only one direction at a time.
3. Rotational Latency –
The amount of time taken by the disk to rotate the track when the read/write head comes to exact sector.
```Average rotational latency,
= (1/2) * one rotation time  ```
4. Data Transfer Time –
It is the time taken to transfer the data.
```Data transfer time,
= (Data to be transfer) / (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

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