Rotational delay

A term from computing applicable to rotating storage devices (such as a typical hard disk drive or floppy disk drive[?] but also the older drum memory systems). The rotational delay is the delay caused by having to wait for the portion of the disk (or drum or whatever) to rotate round so that the data you want to access is readable by the read/write head. Not usually applied to tape drives, even though they rotate.

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 you wanted the data from it). 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.

There are other delays associated with accessing rotating media: seek time.