This is computed by averaging the time necessary for all possible seeks. This tends to overestimate actual average seek time because of locality of disk references. The time necessary for the requested sector to rotate under the head. On average, the disk must rotate halfway around to get to a desired sector. Therefore, the two mechanical components, moving the disk arm and waiting for the data to rotate under the head, add to the latency.
Some disks can read data out of order into a buffer, reducing rotational delay for a large transfer.
The transfer time is the time it takes to read or write a sector. It is a function of the block size, rotation speed, recording density and the speed of the electronics. Magnetic disks Other components to latency Note, too, that other parts of the system can add delays. The controller adds 2 ms overhead to each request. This is the product of tracks per inch on a surface and bits per inch on a track.