5 ms·

This seems implausible at first glance. Even making objects flat at the nanometer scale is quite difficult. When you also introduce movement tolerances (e.g. ro

This seems implausible at first glance. Even making objects flat at the nanometer scale is quite difficult. When you also introduce movement tolerances (e.g. rotation disk is not perfectly aligned), this seems quite extraordinary. Can you provide some references?

cyphar 8y ago
(Sheepishly) My only reference is a talk by Bryan Cantrill[1] where he states that an exec at a hard-drive manufacturer stated it was ".8 nanometers". I will try to find a better source than that. EDIT: There is a paper from 2016 which did an analysis in the difference in the flying height of the head during different operations, and it was measured in Angstroms[2]. I couldn't find one that actually gives a precise value of the flying height. There is a 2011 paper which states that some system they were testing allowed for 4-9 nm flying heights[3] which is about half-an-order-of-magnitude larger than the claim -- but that's already 7 years old. [1]: https://youtu.be/fE2KDzZaxvE?t=1551 https://youtu.be/fE2KDzZaxvE?t=1551 [2]: http://iieng.org/images/proceedings_pdf/E0116009.pdf http://iieng.org/images/proceedings_pdf/E0116009.pdf [3]: http://maeresearch.ucsd.edu/callafon/publications/2011/UweIEEETonM.pdf http://maeresearch.ucsd.edu/callafon/publications/2011/UweIE...
- Misdicorl 8y ago
  This is pretty awesome! Thanks for the links.
loeg 8y ago
Disk platters rotating at 7200 rpm are both perfectly smooth and perfectly aligned. The read head is something like a jumbo jet flying a handful of feet above the (perfectly smooth) ground. It's really crazy how close these things are, moving very fast. And why accelerometers are a significant feature.[0] > Can you provide some references? Wikipedia claims[1]: > In 2011, the flying height in modern drives was a few nanometers. and > The "flying height" is constantly decreasing to enable higher areal density. > At 7,200 RPM, the edge of the platter is traveling at over 120 kilometres per hour (75 mph) So you've got a read head flying at 120 km/h => 33 m/s => 33,000,000,000 nm/s at a height of 3nm or less. Picture that! E.g, a 757 typically cruises at 858 km/h => 238 m/s. So picture your 757 flying at an altitude of 21 nm and that's the metaphor, kinda. The read head is a bit smaller than 1/7 of a 757 jet, obviously. [0]: https://en.wikipedia.org/wiki/Head_crash https://en.wikipedia.org/wiki/Head_crash [1]: https://en.wikipedia.org/wiki/Flying_height https://en.wikipedia.org/wiki/Flying_height
- Misdicorl 8y ago
  The claim was hundreds of picometers, which is an order of magnitude smaller than a few nanometers. Literally nothing can be perfectly flat, nor perfectly aligned.
  - blattimwind 8y ago
    It doesn't need to be. The basic idea is that the fly height is self-regulating; if the head goes away from the platter, its "lift" is reduced, so the springiness of the arm forces it back to the platter. If it moves closer to the platter, lift is increased, so it moves away. Similarly the tracks don't have to be perfectly round or concentric, because the lowest-level head control system in the drive actively tracks the head's current disk track on the surface; head movements aren't a "rotate 11.57821° to track 28139123", but rather "track 28139123 is around here somewhere, lets find it".
    - Misdicorl 8y ago
      Yes, I understand how feedback control systems work. The question was whether they really have the fidelity to do it at hundred picometer resolution. I am dubious because this is a pretty incredible feat. This is the length scale at which atom-atom interactions become important. That implies that the crystal lattice structure of both the read/write head and the underlying platter will affect the dynamics of the system!
    - loeg 8y ago
      > Similarly the tracks don't have to be perfectly round or concentric They have to be pretty dang close at 7200+ rpm.
      - Serow225 8y ago
        Nope. Since a long time, tracks are virtual (array of raw track IDs as a function of sector ID when the arm is held "constant" and averaged for X thousand RPMs as part of the per-drive firmware Calibration at the factory).
lightedman 8y ago
"Even making objects flat at the nanometer scale is quite difficult." Lapidary flat laps regularly get down to 125nm fineness (that's 200,000 grit) with very little issue.

cyphar 8y ago

(Sheepishly) My only reference is a talk by Bryan Cantrill[1] where he states that an exec at a hard-drive manufacturer stated it was ".8 nanometers". I will try to find a better source than that. EDIT: There is a paper from 2016 which did an analysis in the difference in the flying height of the head during different operations, and it was measured in Angstroms[2]. I couldn't find one that actually gives a precise value of the flying height. There is a 2011 paper which states that some system they were testing allowed for 4-9 nm flying heights[3] which is about half-an-order-of-magnitude larger than the claim -- but that's already 7 years old. [1]: https://youtu.be/fE2KDzZaxvE?t=1551 https://youtu.be/fE2KDzZaxvE?t=1551 [2]: http://iieng.org/images/proceedings_pdf/E0116009.pdf http://iieng.org/images/proceedings_pdf/E0116009.pdf [3]: http://maeresearch.ucsd.edu/callafon/publications/2011/UweIEEETonM.pdf http://maeresearch.ucsd.edu/callafon/publications/2011/UweIE...

Misdicorl 8y ago
This is pretty awesome! Thanks for the links.

loeg 8y ago

Disk platters rotating at 7200 rpm are both perfectly smooth and perfectly aligned. The read head is something like a jumbo jet flying a handful of feet above the (perfectly smooth) ground. It's really crazy how close these things are, moving very fast. And why accelerometers are a significant feature.[0] > Can you provide some references? Wikipedia claims[1]: > In 2011, the flying height in modern drives was a few nanometers. and > The "flying height" is constantly decreasing to enable higher areal density. > At 7,200 RPM, the edge of the platter is traveling at over 120 kilometres per hour (75 mph) So you've got a read head flying at 120 km/h => 33 m/s => 33,000,000,000 nm/s at a height of 3nm or less. Picture that! E.g, a 757 typically cruises at 858 km/h => 238 m/s. So picture your 757 flying at an altitude of 21 nm and that's the metaphor, kinda. The read head is a bit smaller than 1/7 of a 757 jet, obviously. [0]: https://en.wikipedia.org/wiki/Head_crash https://en.wikipedia.org/wiki/Head_crash [1]: https://en.wikipedia.org/wiki/Flying_height https://en.wikipedia.org/wiki/Flying_height

Misdicorl 8y ago
The claim was hundreds of picometers, which is an order of magnitude smaller than a few nanometers. Literally nothing can be perfectly flat, nor perfectly aligned.
- blattimwind 8y ago
  It doesn't need to be. The basic idea is that the fly height is self-regulating; if the head goes away from the platter, its "lift" is reduced, so the springiness of the arm forces it back to the platter. If it moves closer to the platter, lift is increased, so it moves away. Similarly the tracks don't have to be perfectly round or concentric, because the lowest-level head control system in the drive actively tracks the head's current disk track on the surface; head movements aren't a "rotate 11.57821° to track 28139123", but rather "track 28139123 is around here somewhere, lets find it".
  - Misdicorl 8y ago
    Yes, I understand how feedback control systems work. The question was whether they really have the fidelity to do it at hundred picometer resolution. I am dubious because this is a pretty incredible feat. This is the length scale at which atom-atom interactions become important. That implies that the crystal lattice structure of both the read/write head and the underlying platter will affect the dynamics of the system!
  - loeg 8y ago
    > Similarly the tracks don't have to be perfectly round or concentric They have to be pretty dang close at 7200+ rpm.
    - Serow225 8y ago
      Nope. Since a long time, tracks are virtual (array of raw track IDs as a function of sector ID when the arm is held "constant" and averaged for X thousand RPMs as part of the per-drive firmware Calibration at the factory).

lightedman 8y ago

"Even making objects flat at the nanometer scale is quite difficult." Lapidary flat laps regularly get down to 125nm fineness (that's 200,000 grit) with very little issue.