What Is the Latency on Hard Drives?

The spinning disk in a hard drive creates a brief delay called latency.

Your computer's hard drive is a mechanical system that operates by reading data from a flat disk platter spinning at high speed. Typically, a short delay occurs as the drive searches the rotating disk for particular data. Computer scientists refer to this brief delay as rotational latency.

Disk Rotation

In the hard drive, a magnetic read-write head rides on a finger-like mechanism just above a spinning metallic disk. When the computer requests data from the disk, it has to wait for the data on the disk to "come around" to the head's position.

Latency Time

The faster the disk spins, the shorter the wait for the computer. A hard drive spinning at 7,200 rotations per minute has an average latency of 4.2 thousandths of a second. This is approximately one-half the time it takes for the disk to turn one revolution.

Latency Consequences

While 4.2 thousandths of a second seems inconsequential, a computer might process many thousands of data requests. The computer's CPU is fast enough to keep up with the workload but the hard disk's rotational latency creates a bottleneck.

References

• PC Guide: Latency
• Search Storage: "What to look for when buying disk storage;" Bilal Mehdi; August 2009