access time

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Access time is the time delay or latency between a request to an electronic system, and the access being completed or the requested data returned.

• In a telecommunications system, access time is the delay between the start of an access attempt and successful access. Access time values are measured only on access attempts that result in successful access.
• In a computer, it is the time interval between the instant at which an instruction control unit initiates a call for data or a request to store data, and the instant at which delivery of the data is completed or the storage is started.
• In disk drives, disk access time is the time required for a computer to process data from the processor and then retrieve the required data from a storage device, such as a hard drive. For hard drives, disk access time is determined by a sum of the spin-up time, seek time, rotational delay and transfer time.
  • Spin-up time - is the time required to accelerate the disk to operating speed. Frequently used drives are often left spinning to improve access time, but drives may be spun down to reduce energy use or noise.
  • Seek time - is the time for the access arm to reach the desired disk track.
  • Rotational delay - the delay for the rotation of the disk to bring the required disk sector under the read-write mechanism. It greatly depends on rotational speed of a disk, measured in revolutions per minute (RPM).
  • Transfer time - time during which data is actually read or written to medium, with a certain throughput.

Theoretical averages of the access time or latency are shown in the table below, based on the empirical relation that the average latency in milliseconds for such a drive is about 30000/RPM:

Seek time: This is the amount of time between when the CPU requests a file and when the first byte of the file is sent to CPU. Times between 10 to 20 milliseconds are common.

Spindle RPM	Average latency (ms)
4200	7.14
5400	5.55
7200	4.17
10000	3
15000	2

See also

• Memory latency
• Rotational delay
• Seek time
• Comparison of latency and bandwidth
• Throughput
• Lead time

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).