Rotational delay

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Rotational delay or rotational latency is the time required for the addressed area of a computer's disk drive (or drum) to rotate into a position where it is accessible by the read/write head. The term applies to rotating storage devices (such as a hard disk or floppy disk drive, and to the older drum memory systems).

Rotational delay is one of the three delays associated with reading or writing data on a disk, and somewhat similar for CD or DVD drives. The others are seek time and transfer time. Sometimes spin-up time is considered as an independent fourth factor though it is a rotational delay that only affects systems that automatically spin down (turn off) drives to conserve power, noise, etc..

Maximum rotational delay is the time it takes to do a full rotation excluding any spin-up time (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, 5400 or 4200 revolutions per minute were typical hard disk drive spindle speeds; at 7200 rpms, 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.

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