Rotationslatens

Rotationslatens, often called rotational latency in English, is the time a spinning hard disk drive (HDD) requires for the correct sector to rotate under the read/write head after a read or write request is issued. It is a core component of the overall disk access latency and is inherent to the mechanical design of HDDs.

The latency depends primarily on the disk’s rotational speed, measured in revolutions per minute (RPM), and on where the requested data is located on the disk. For a random access pattern, the average rotationslatens equals half a full rotation. The full rotation period is 60 divided by the RPM, so the average rotationslatens is 30 divided by the RPM in seconds. For example, a 7200 RPM drive has an average rotationslatens of about 4.17 ms, while a 5400 RPM drive averages about 5.56 ms. The maximum rotationslatens equals the full rotation period (e.g., 8.33 ms at 7200 RPM).

Rotationslatens interacts with seek time and data transfer time to determine total access time. Caching, data locality, and drive prefetching can mitigate perceived latency for sequential reads, but random access often incurs higher delays. Modern HDDs may also employ caches and planning algorithms to hide some rotational delays, though the mechanical nature of the delay cannot be eliminated.

In contrast, solid-state drives (SSDs) exhibit negligible rotationslatens because they have no spinning platters. For performance analysis, rotationslatens is treated alongside seek time and transfer rate to model disk I/O behavior and to guide storage system design and tuning.