Rotational delay

Rotational delay or rotational latency is one of the three delays associated with reading or writing data on a computer's disk drive, and somewhat similar for CD or DVD drives. The others are seek time and transfer time. The term applies to rotating storage devices (such as a hard disk or floppy disk drive, and to the older drum memory systems). The rotational delay is the time required for the addressed area of the disk (or drum) to rotate into a position where it is accessible by the read/write head.

Maximum rotational delay is the time it takes to do a full rotation (as the relevant part of the disk may have just passed the head when the request arrived). Most rotating storage devices rotate at a constant angular rate (constant number of revolutions per second). The maximum rotational delay is simply the reciprocal of the rotational speed (appropriately scaled). In 2001, 7200 revolutions per minute is typical for a hard disk drive; its maximum rotational delay will be 60/7200 s or about 8 ms.

Average rotational delay is also a useful concept - it is half the maximum rotational delay.

See also

• Seek time
• Access time
• Hard disk