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

Seek time:

  • The seek time of a hard disk measures the amount of time required for the read/write heads to move between tracks over the surfaces of the platters.

  • Seek time is normally expressed in milliseconds (commonly abbreviated "msec" or "ms"), with average seek times for most modern drives today in a rather tight range of 8 to 10 ms.

Different types of seeks:

  • Average: As discussed, this is meant to represent an average seek time from one random track (cylinder) to any other. This is the most common seek time metric, and is usually 8 to 10 ms, though older drives had much higher numbers, and top-of-the-line SCSI drives are now down to as low as 4 ms!

  • Track-to-Track: This is the amount of time that is required to seek between adjacent tracks. This is similar in concept (but not exactly the same as) the track switch time and is usually around 1ms. (Incidentally, getting this figure without at least two significant digits is pretty meaningless; don't accept "1 ms" for an answer, get the number after the decimal point! Otherwise every drive will probably round off to "1 ms".)

  • Full Stroke: This number is the amount of time to seek the entire width of the disk, from the innermost track to the outermost. This is of course the largest number, typically being in the 15 to 20 ms range. In some ways, combining this number with the average seek time represents the way the drive will behave when it is close to being full.

Rotational Latency:

  • Rotational latency is the delay waiting for the rotation of the disk to bring the required disk sector under the read-write head.

  • It depends on the rotational speed of a disk (or spindle motor), measured in revolutions per minute(RPM).For most magnetic media-based drives, the average rotational latency is typically based on the empirical relation that the average latency in milliseconds for such a drive is one-half the rotational period.

Two types of disk rotation methods:

1) constant linear velocity (CLV), used mainly in optical storage, varies the rotational speed of the optical disc depending upon the position of the head, and

2) Constant angular velocity (CAV), used in HDDs, standard FDDs, a few optical disc systems, and vinyl audio records, spins the media at one constant speed regardless of where the head is positioned.

  • To access data, the actuator arm moves the R/W head over the platter to a particular track while the platter spins to position the requested sector under the R/W head. The time taken by the platter to rotate and position the data under the R/W head is called rotational latency. This latency depends on the rotation speed of the spindle and is measured in milliseconds. 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. Average rotational latency is around 5.5 ms for a 5,400-rpm drive, and around 2.0 ms for a 15,000-rpm drive.


  • Utilization factor or use factor is the ratio of the time that a piece of equipment is in use to the total time that it could be in use. It is often averaged over time in the definition such that the ratio becomes the amount of energy used divided by the maximum possible to be used.

  • Utilizationcan be defined as the ratio of the service time to the average inter-arrival time, and is expressed as: U = RS /Ra Where “RS“is the service time, or the average time spent by a request on the controller.
    1/RS is the service rate.

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