4 ms·

> skates of magnetic readers flying on a cushion of air above a rapidly rotating disc, with the gap separating a few dozen nanometers, often smaller than the pr

by cyphar 8y ago

> skates of magnetic readers flying on a cushion of air above a rapidly rotating disc, with the gap separating a few dozen nanometers, often smaller than the process size in the controller's silicon. Complete aside, the fly-height of a magnetic head is actually fractions of a nanometer (i.e. hundreds of picometers). EDIT: I got this from a talk by Bryan Cantrill[1]. The fly-height is allegedly 0.8 nanometers (800 picometers). [1]: https://youtu.be/fE2KDzZaxvE?t=1551 https://youtu.be/fE2KDzZaxvE?t=1551

DoctorOetker 8y ago
I didnt know they where that close, isn't that similar distances as an scanning tunneling microscope tip to an object? or even closer? Could you provide a reference to a scientific article stating this?
Misdicorl 8y ago
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!
- 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.
hwillis 8y ago
I do not believe that. 1. I can't find a source that says less than a few nanometers. 2. 300 picometers is roughly the diameter of a helium diatom. The head cannot possibly float through hydrodynamic means if an air molecule can barely even fit under it.
- dogecoinbase 8y ago
  Neat, you independently discovered why the industry is moving to helium-filled drives.
- cyphar 8y ago
  Bryan Cantrill made this claim in a talk recently[1], and the fly-height is apparently 800 picometers. [1]: https://youtu.be/fE2KDzZaxvE?t=1551 https://youtu.be/fE2KDzZaxvE?t=1551
- blattimwind 8y ago
  > The head cannot possibly float through hydrodynamic means if an air molecule can barely even fit under it. It can. Since siblings liked airplane analogies, here is another one: Consider the head to be an airplane. It has somewhat wing-similar features which provide a lifting force, but the actual read/write head sits below those features (like, say, a landing gear is below wings).
  - Serow225 8y ago
    This is correct :)
- the8472 8y ago
  You can at least fit several layers of iron atoms into that gap, assuming they're part of the crystal lattice. So the surface does not have to be perfectly smooth. https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page)#Metallic_radius https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(...

by cyphar 8y ago

DoctorOetker 8y ago

I didnt know they where that close, isn't that similar distances as an scanning tunneling microscope tip to an object? or even closer? Could you provide a reference to a scientific article stating this?

Misdicorl 8y ago

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!
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.

hwillis 8y ago

I do not believe that. 1. I can't find a source that says less than a few nanometers. 2. 300 picometers is roughly the diameter of a helium diatom. The head cannot possibly float through hydrodynamic means if an air molecule can barely even fit under it.

dogecoinbase 8y ago
Neat, you independently discovered why the industry is moving to helium-filled drives.
cyphar 8y ago
Bryan Cantrill made this claim in a talk recently[1], and the fly-height is apparently 800 picometers. [1]: https://youtu.be/fE2KDzZaxvE?t=1551 https://youtu.be/fE2KDzZaxvE?t=1551
blattimwind 8y ago
> The head cannot possibly float through hydrodynamic means if an air molecule can barely even fit under it. It can. Since siblings liked airplane analogies, here is another one: Consider the head to be an airplane. It has somewhat wing-similar features which provide a lifting force, but the actual read/write head sits below those features (like, say, a landing gear is below wings).
- Serow225 8y ago
  This is correct :)
the8472 8y ago
You can at least fit several layers of iron atoms into that gap, assuming they're part of the crystal lattice. So the surface does not have to be perfectly smooth. https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(data_page)#Metallic_radius https://en.wikipedia.org/wiki/Atomic_radii_of_the_elements_(...