What defines the time taken to position the read/write head across the platter with a radial
movement in a disk drive?

A.
Seek time

B.
Rotational latency

C.
Data transfer time

D.
Service time

Explanation:
Seek Time
The seek time (also called access time) describes the time taken to position the R/W heads across
the platter with a radial movement (moving along the radius of the platter). In other words, it is the
time taken to position and settle the arm and the head over the correct track.
Therefore, the lower the seek time, the faster the I/O operation. Disk vendors publish the following
seek time specifications:
Full Stroke: The time taken by the R/W head to move across the entire width of the disk, from the
innermost track to the outermost track.
Average: The average time taken by the R/W head to move from one random track to another,
normally listed as the time for one-third of a full stroke.
Track-to-Track: The time taken by the R/W head to move between adjacent tracks.
Each of these specifications is measured in milliseconds. The seek time of a disk is typically
specified by the drive manufacturer. The average seek time on a modern disk is typically in the
range of 3 to 15 milliseconds. Seek time has more impact on the I/O operation of random tracks
rather than the adjacent tracks. To minimize the seek time, data can be written to only a subset of
the available cylinders. This results in lower usable capacity than the actual capacity of the drive.
For example, a 500-GB disk drive is set up to use only the first 40 percent of the cylinders and is
effectively treated as a 200-GB drive. This is known as short stroking the drive.
EMC E10-001 Student Resource Guide. Module 2: Data Center Environment