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)?
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.
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
Your calculation is the average track seek, you need to add the sector seek to that.
When seeking for a read operation, the head is positioned on (a) a track, at a given (b) sector. The (average) seek time is the time taken to switch to that position to any other position, with both (a) track and (b) sector.
When positioned, the read can start.
The disk RPM is into play for this, if it spins at 600rpm and has 100 sectors per track, it means that it seeks sectors at
60000ms (because rpm = per minute)
/
600rpm (disk spin speed)
/
100sectors (per track)
=
1ms (to change from a sector to the next adjacent one)
Normally, you would have to consider that as you change tracks, the disk is still spinning and thus account for the sector offset change. But since we are interested only in the average, this cancels out (hopefully).
So, to your 249.5 ms for the track seek average time, you need to add :
same formula :
sum 0->100/100 * 1ms (sector seek speed) = 50.5ms
Thus, the average seek speed for both track and sector is 300ms.
来源：https://stackoverflow.com/questions/41767414/how-is-average-seek-time-calculated