Rotation latency, also known as rotational delay, is the time it takes for the desired sector on a spinning hard disk drive (HDD) to rotate under the read/write head. It's a key factor contributing to overall disk access time.
Understanding Rotational Latency
HDDs store data on circular platters that spin at a constant speed. To access a specific piece of data, the read/write head needs to be positioned over the correct track (seek time) and then wait for the desired sector to rotate under the head (rotational latency).
Rotational latency depends directly on the drive's rotational speed, measured in revolutions per minute (RPM). A higher RPM means a shorter average rotational latency.
Calculating Average Rotational Latency
The average rotational latency can be calculated using the following formula:
Average Rotational Latency = (60 / RPM) / 2
Where:
- RPM is the rotational speed of the disk (revolutions per minute).
- 60 converts RPM to revolutions per second.
- Dividing by 2 gives the average latency, as the desired sector could be anywhere on the platter.
For example:
| RPM | Calculation | Average Rotational Latency (ms) |
|---|---|---|
| 5400 | (60 / 5400) / 2 = 0.005556 seconds | 5.56 |
| 7200 | (60 / 7200) / 2 = 0.004167 seconds | 4.17 |
| 10000 | (60 / 10000) / 2 = 0.003 seconds | 3.00 |
| 15000 | (60 / 15000) / 2 = 0.002 seconds | 2.00 |
As you can see, faster RPM drives have lower rotational latency.
Impact on Performance
Rotational latency contributes significantly to the overall time it takes to read or write data to a hard drive. While seek time and transfer rate also play roles, minimizing rotational latency is crucial for improving disk performance, especially for applications that require frequent random access to data. Solid-state drives (SSDs) eliminate rotational latency entirely, which is a major reason for their superior performance compared to HDDs.
Summary
Rotational latency is a critical performance metric for hard disk drives, representing the delay caused by the disk's rotation while waiting for the desired data sector to reach the read/write head. Reducing rotational latency improves overall disk access time and system responsiveness.
