Seek time, rotational latency and utilization in case of disk drive

Seek drive - (also called access time)

The time taken to position the R/W heads across the platter with a radial movement (moving along 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 stroke - The time taken by the R/W head to move across the entire width of the disk ,from the innermkst 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 stroke.

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 as called rotational latency.This latency depends on the rotaion speed of the spindle & is measured in millisecond.The average rotational latency is one half of the time taken for a full roration.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 or adjacent sectors.

Eg.

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

$V=\frac{R_{B}}{R_{a}}$

where $R_{8}$= Service time

      $R a$ = average inter-arrival time

&

$R a=\frac{1}{a}$ where a = arrival time

Eg -