What is meant by seek time, rotational latency and utilization in case of disk drive

Seek time :- (also called access time)

  • The time taken to position the R/W heads across the platter with a radial movement (moving aong the radius of the platter) i.e the time taken to reposition & settle the arm & the head over the correct track.

Following are the seek time specifications :-

a) Full risks :- The time taken by the R/W head to move across the entire width of disk, from the innermost track to the outermost track.

b) 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 stoke.

c) Track-to-track :- The time taken by the R/W head to move between adjacent tracks.

  • measured in milliseconds.

  • Rotational latency :- The time taken by the platter to rotate & position the data under the R/W head is called latency.

  • This latency depends on the rotation speed of the spindle & is measured in millisecond.

  • The average rotational latency is one-half of the time taken for a full rotation.

  • Similar to the seek time, rotational latency has more impact on the reading/writing of random sectors on the disk than on the same operations on adjacent sectors.

Eg :-

Rotational latency for 5400 rpm drive is Avg Rotational latency = 1/RPM/60 X 0.5 X 1000

= 30000/RPM

= 5.5 ms

Utilization law:-

  • Utilization can defined as the ratio of the service time to the average inter-arrival time & is expressed as


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