access time

Disk time is the time delay or latency between a request for clock to an electronic system, and the access being granted 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 magnetic disk drives, it is the time for the access arm to reach the desired track and the delay for the rotation of the disk to bring the required sector under the read-write mechanism. 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 : It 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

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