Noob hdd read surface test question

[dorsalis]

I'm performing a read surface test to see if there are any bad blocks or sectors on my 3tb hdd. Why does performance (MB/s) decline with increasing position (GB)??

Thanks for your time.

 

[bucknutty]

This not really a noob question, it is actually pretty complex. I dont have an exact answer for you, hopefully there is someone on here that has a more complete understanding of hdds than me.

Every track on the HDD holds the same amount of info. So the smaller inside tracks are more dense than the larger outside tracks. Because the info on the inside is closer together it gets moved under the head faster and read faster. As the seek head moves to the outside of the disk the data is less dense and further apart so the performance is lower.

Generally because the inner tracks are closer to the center the seek head has to move less so seek times are lower.

However because the outside edge of the disk spins faster than the inside the info on the outer edges are spun under the head faster so often times hdd benchmarks show the outer edge as having a faster read write.

Here the seek arm has to move the most to read the data at the outside edge so the seek times are normally higher.

My guess is the lower perfomance is due to the increased seek time of the head moving more as it gets closer to the outside edge of the disk.

 

[fzabkar]

Each track on modern hard drives has the same data density, ie the same number of bits per inch. Therefore the outermost tracks have more data per track than the innermost tracks. This means that less data passes under the head during each revolution as you move from the outer tracks to the inner ones. The ratio between the diameters of the outermost and innermost tracks is about 2:1, so the maximum and minimum data transfer rates will also be in the same proportion.

See http://en.wikipedia.org/wiki/Zone_bit_recording

... and http://hddscan.com/doc/HDD_Tracks_and_Zones.html

 

[spawnkiller]

If the disk spins at 7200rpm the actual start position (the further from the center) 1 rotation have more data as the actual circumference is higher and reduce each time you go closer to the center...

In clear 1 rpm on the start is maybe 3-4 rpm on the center so with 3-4 rpm at the start it's way faster, that's why HDD are slower on the end of the partition...