Understanding Disk Rotational Delay Calculation

The question asks for the maximum rotational delay of a disk platter rotating at a specific speed. Rotational delay, also known as rotational latency, is the time it takes for the desired sector on the disk track to rotate under the read/write head.

The maximum rotational delay is the time it takes for the disk platter to complete one full revolution. This happens when the head has just arrived at the correct track, but the desired sector is just passing the head, requiring a full rotation before it comes around again.

Calculating Rotational Delay for 5400 RPM

We are given that the disk platters rotate at 5400 rpm (revolutions per minute).

To find the time for one revolution, we first need to convert the speed from revolutions per minute to revolutions per second.

• Speed = 5400 revolutions per minute
• Convert to revolutions per second: Speed in rps = $\frac{5400 \text{ revolutions}}{1 \text{ minute}} \times \frac{1 \text{ minute}}{60 \text{ seconds}}$
• Speed in rps = $\frac{5400}{60}$ revolutions per second
• Speed in rps = 90 revolutions per second

The time taken for one revolution is the inverse of the speed in revolutions per second.

• Time per revolution = $\frac{1}{\text{Speed in rps}}$
• Time per revolution = $\frac{1}{90}$ seconds

The maximum rotational delay is equal to the time taken for one full revolution.

• Maximum Rotational Delay = $\frac{1}{90}$ seconds

Evaluating the Calculation

Let's calculate the value of $\frac{1}{90}$ seconds:

$\frac{1}{90} \approx 0.01111...$ seconds

Comparing this value to the given options:

• 0.011 seconds
• 0.11 seconds
• 0.0011 seconds
• 1.1 seconds

The calculated value of approximately 0.0111 seconds is closest to 0.011 seconds.

Therefore, the maximum rotational delay is approximately 0.011 seconds.

Revision Table: Disk Access Time Components

Additional Information: Disk Performance Metrics

Understanding disk performance involves looking at different factors that contribute to the time it takes to access data. The total time to access data on a hard disk is roughly the sum of seek time, rotational delay, and transfer time.

• Average Seek Time: This is the average time taken for the read/write heads to move from one random track to another. The question states an average seek time of 10 milliseconds for this specific disk.
• Average Rotational Delay: While the maximum rotational delay is the time for a full rotation, the average rotational delay is typically half of the maximum time, assuming the head arrives at the track randomly relative to the sector's position.
• Transfer Time: This is determined by the amount of data being transferred and the disk's data transfer rate. It's the time spent actually reading or writing the bits once the head is positioned correctly.

Higher RPM means lower rotational delay and faster data access, assuming other factors are equal. For example, a disk rotating at 7200 rpm would have a shorter rotational delay than one at 5400 rpm.