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Old 04-10-2011, 02:05 PM   #31
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Ok, I've read some more whitepapers about modern HDDs' geometry and that what I've got from there. CHS addresses don't reflect the real disk geometry now. So I've thought up a tricky workaround.

I suppose that a number of sectors on each side of each plate in HDD is approximately the same. I need a number of a sector that's located approximately in the middle of a side of plate. In other words a distance from this sector to a hub is equal to one half of a distance from the hub to a border of the plate. One sector occupies some area of the plate. All sectors on one side of plate occupy all this side. A circle with a radius equal to one half of a radius of the plate would occupy a quarter of the side of plate. So one quarter of all sectors on the side of plate would be in this circle. Needed sector is somewhere near of a border of this circle. First sector on the side of plate is at it's border. So number of the needed sector is about 3/4*N (N is a number of sectors on the side of plate).

r2 on attached picture is the radius of the plate. r1 is the radius of the circle. r2 == 2*r1.

S1 - area of the circle
S1 == pi*r1^2

S2 - area of the side of plate
S2 == pi*r2^2 == pi*(2*r1)^2 == 4*pi*r1^2 == 4*S1

S1 == 1/4*S2

Is this acceptable way to search the middle of plate?
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Old 04-10-2011, 03:07 PM   #32
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To be honest, I don't think it would matter much anymore. Drives are so much faster nowadays plus memory is a lot cheaper. If you really need to worry about speed that much, more ram would be a better solution. I just built a new rig and I got 16Gbs in mine. It was only about $200.00 if I recall correctly. I just caught it on sale.

Also, swap is slow pretty much anywhere. That should be the last thing you want used.

Old 04-11-2011, 06:07 AM   #33
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Please post links to the whitepapers you have found, it's hard for the rest of us to discuss this w/o the same background info. you have available. My memory of my research into this subject says that your: "I suppose that a number of sectors on each side of each plate in HDD is approximately the same." is an acceptable simplification.

There are 2 things are wrong w/ your method:
  1. I have taken drives apart & measured, reality is: r2 ~= 2.71*r1 (see below)
  2. You do not follow through to find "r50", the median radius -- the one at which S2-S50 == S50-S1
I have an actual platter in my hand as write. There are 2 (inner & outer) unwritten-on bands between the physical edges (of the bearing & platter), & the edges of the magnetic info. These are necessary because the drive's heads can't go all the way to the physical edges of the platter or bearing. These bands seem to be 1/16" wide, changing any diameters by 1/8". That's 2 more radii or diameters to consider. Altogether:
Dh (bearing) ==  1-3/16" == 1.1875" == 3.01625 cm
D1 (info. 1) ~=  1-5/16" == 1.3125" == 3.33375 cm
D2 (info. 2) ~=  3-9/16" == 3.5625" == 9.04875 cm
Dp (platter) == 3-11/16" == 3.6875" == 9.36625 cm

dalek is probably right -- just buy more RAM.

However, the fact is that info. transfers are ~2.7 times faster from the outer sectors of a drive than from the inner sectors of a drive, & this will remain true for as long as we continue to use drives w/ platters. I think that balancing the speed of transfer against against head movement time is an interesting intellectual -- but not practical -- problem. My seat of the pants guess is that the optimum place for swap is near R75. I usually implement this on a single drive system by making it the 3rd primary partition, just before the extended partition.

If you're not going to buy more RAM, then put your swap on the outer sectors of a separate spindle. If you want to get really fanatical about it, track down an obsolete SCSI drive, cable, & card that will give you reasonable speed. See this chart of Parallel SCSI interfaces. Ultra2 SCSI, aka Fast-40, or better would probably do the trick.


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