How is Average Seek Time Calculated?

Question

A hard disk system has the following parameters :

Number of tracks = 500
Number of sectors/track = 100
Number of bytes /sector = 500
Time taken by the head to move from one track to adjacent track = 1 ms
Rotation speed = 600 rpm.

What is the average time taken for transferring 250 bytes from the disk ?

Well I wanted to know How the average seek time is calculated ?

My Approach

Avg. time to transfer = Avg. seek time + Avg. rotational delay + Data transfer time

Avg Seek Time

given that : time to move between successive tracks is 1 ms

time to move from track 1 to track 1 : 0ms

time to move from track 1 to track 2 : 1ms

time to move from track 1 to track 3 : 2ms

..

..

time to move from track 1 to track 500 : 499 ms

Avg Seek time = = 249.5 ms

But After Reading Answer given here Why is average disk seek time one-third of the full seek time?

Im confused with my approach.

My question is

Is my Approach Correct ?

• If not Please explain the correct way to calculate Average seek time
• If Yes please explain wh we are not considering average for every possible pair of tracks (as mentioned in the above link)?

Answer 1

There are a lot more than 500 possible seek times. Your method only accounts for seeks starting at track 1.

What about seeks starting from track 2? Or from track 285?

I wouldn't say your approach is wrong, but it's certainly incomplete.

Answer 2

As is pointed out in the link you're reffering to in this question the average time is calculated as average distance from ANY track to ANY track. So you have to add all of the Subsums to the one you are using to calculate average seek time and then divide this sum by the number of tracks. It sums out to: N/3, where N is the distance between track 0 and last. f.eg. average distance from track 249 to ANY other track is:middle average sum