Access time

For access time in Unix file statistics, see stat (Unix).

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 disk drives, disk access time is determined by a sum of the spin-up time, seek time, rotational latency, and transfer time.
  • Spin-up time - is the time required to accelerate the disk to operating speed. Most drives are left spinning to improve access time, but drives may be spun down to reduce energy use or noise as for example in laptop computers.
  • Seek time - is the time for the actuator arm to reach the desired disk track.
  • Rotational latency - 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.

In disk drive art the term Average Access Time usually refers to average seek time.

See also

• Memory latency
• Mechanical latency
• rotational latency
• 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).