Dave AndersonAug 12, 2025

Digging into the drive in my NAS that faulted, I'm reminded that magnetic hard drives are preposterously magical technology.

Case in point, using Seagate's tools I can get the drive to tell me how much it's adjusted the fly height of each of its 18 heads over the drive's lifetime, to compensate for wear and stuff. The drive provides these numbers in _thousandths of an angstrom_, or 0.1 _picometers_.

For reference, one helium atom is about 49 picometers in diameter. The drive is adjusting each head individually, in increments of a fraction of a helium atom, to keep them at the right height. I can't find numbers of modern drives, but what I can find for circa ten years ago is that the overall fly height had been reduced to under a nanometer, so the drive head is hovering on a gas bearing that's maybe 10-20 helium atoms thick, and adjusting its position even more minutely than that

This is _extremely_ silly. You can buy a box that contains not just one, but several copies of a mechanism capable of sub-picometer altitude control, and store shitposts on it! That's wild.

Anyway my sad drive apparently looks like it had a head impact, not a full crash but I guess clipped a tiny peak on the platter and splattered a couple thousand sectors. Yow. But I'm told this isn't too uncommon, and isn't the end of the world? Which is, again, just ludicrous to think of. The drive head that appears to have bonked something has adjusted its altitude by almost 0.5 picometers in its 2.5 years in service. Is that a lot? I have no idea!

Aside from having to resilver the array and the reallocated sector count taking a big spike, the drive is now fine and both SMART and vendor data say it could eat this many sectors again 8-9 times before hitting the warranty RMA threshold. Which is very silly. But I guess I should keep an eye on it.

Show threadSludgeAug 12, 2025
@danderson chances are the engineers picked those units "just to be safe for a while", not necessarily because that resolution is what the drive is actually capable of adjusting in
Show threadMajor Denis BloodnokAug 12, 2025
@SludgePhD @danderson Yes, this can't possibly be right - the head could at best only be manufactured to a precision of one atom's-width, it can only wear in atom's-width lumps, how could it possibly (or usefully) be adjusted in fractions of that?
Show threadRichard BrockieAug 16, 2025

@denisbloodnok @SludgePhD @danderson

Those same observations/arguments were made within the industry, but the data are clear that we can adjust spacing with pm as the unit. HDDs are indeed magical devices, just like the OP said!

Show threadMajor Denis BloodnokAug 16, 2025
@RichardBrockie Please can we _see_ some of this data? (In particular, "with pm as the unit" I think reasonably covers anything up to three orders of magnitude less precise than the original claim.)
Show threadRichard BrockieAug 16, 2025

@denisbloodnok

Much is confidential. Here’s a recent paper from a collaboration between some of my colleagues and UC Berkeley.

Cheng, Q., Rajauria, S., Schreck, E. et al.

In-situ sub-angstrom characterization of laser-lubricant interaction in a thermo-tribological system.

Commun Eng 3, 138 (2024). https://doi.org/10.1038/s44172-024-00284-3

Show threadMajor Denis BloodnokAug 16, 2025
@RichardBrockie Perhaps I'm missing something but this appears to be about _measuring_ lubricant thickness at, well, as they say ~0.2 Å resolution. That's pretty tiny but it doesn't seem to have much bearing on the question of whether the drive head can be moved in increments 200 times smaller (ie, much smaller than an atom) or what good that would do.
Show threadRichard Brockie

@denisbloodnok

As I said before, much data is confidential, but our colleagues at Seagate have indicated their unit in their SMART data.

This paper is at the same scale: 0.2A is 20pm. Elsewhere in the thread it was mentioned how we control the fly height. The paper indicates some of the reasons we need to control the fly height.

As to how we measure spacing, here’s a good starting point. http://dx.doi.org/10.1109/TMAG.2011.2157672

Aug 16, 2025 at 6:05pmWeb
Show threadMajor Denis BloodnokAug 16, 2025

@RichardBrockie It's not at the same scale (or about positioning the head). The original claim here was for 0.1pm, 200 times as small.

The second paper appears to be (as you say) about measuring spacing not moving the head in 0.1pm increments, and while I can't read the whole thing the abstract says "a repeatability of less than 0.1 nm was obtained".

So far you've shown me nothing to justify the original claim.

Show threadRichard BrockieAug 17, 2025

@denisbloodnok

If you read what I wrote, it is that I’m agreeing on the units that STX have published for spacing. I’ll point out that Marchon et al. is from 11 years ago and the full pdf is available.

Tribological understanding has progressed. If you’re genuinely interested I’ve shared good starting points for a literature search.

Show threadMajor Denis BloodnokAug 17, 2025
@RichardBrockie If what you mean is that the use of "picometres" is not unreasonable for any figure at all in this space, great. You'll forgive me for being a little confused because, since I was specifically dubious about the claim that the head can move in 0.1 pm increments, I expected you to be saying something in support of that claim and was examining what you were saying on that basis.
Show threadMajor Denis BloodnokAug 17, 2025

@RichardBrockie
> Tribological understanding has progressed.

Has it progressed to the point of understanding I'm sceptical of a claim about moving the head, not lubricating it?