Mastodawn

Dave Anderson Aug 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.

@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

Major Denis Bloodnok Aug 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?

Dave Anderson Aug 12, 2025

@denisbloodnok @SludgePhD The drive head is mounted on servos that can be adjusted through deflection, not just moving up/down by increments of one atom. That doesn't limit you to integer atoms.

Wear is also not necessarily mechanical, there's a lot of complex electronics in the pickup mechanism that reads the analog signal off the platter and amplifies/cleans it to recover the bit data. The helium pressure in the case also drops over time, which is going to change the properties of the gas bearing the head is riding on. For any number of reasons, minutely adjusting the head's position may result in significant changes to the head's ability to read the data on the track.

Dave Anderson Aug 12, 2025

@denisbloodnok @SludgePhD Other fun things you can find in the vendor's debug metrics on the drive: the signal strength of the head's writes changes as the drive head ages, as does the physical position of where the signal gets laid on the platter. The drive includes per-head adjustments for exactly how to do the writes, as well as track position offsets to keep the data in the right place. Modern drives are a giant bag of active closed loop feedback systems keeping everything in place as the component characteristics change minutely with age.

Dave Anderson Aug 12, 2025

@denisbloodnok @SludgePhD It's also worth asking: why would they lie? They don't have to expose any of this data, it's not part of standard SMART data, it's only accessible through vendor-specific commands. They could just not publish tools to read them, and not document what the values mean. Instead they open-sourced the software to extract this data from the drives, and documented specifically what each value is measuring. Why go to all that trouble and then lie about what the values mean?

Vassil Nikolov | Васил Николов Aug 12, 2025

I can only note that these technologies have been developed for, very roughly, about three quarters of a century and under high pressure to improve.
This is a very long sequence of changes and it can go very far.

@danderson @denisbloodnok @SludgePhD

Major Denis Bloodnok Aug 12, 2025

@danderson @SludgePhD Deflection just means you're trying to make an even tinier adjustment somewhere else.

The head is far _bumpier_ than the supposed amount of adjustment here, and so is the surface of the drive.

Dave Anderson Aug 12, 2025

@denisbloodnok @SludgePhD that's not how the geometry of the drive heads works, but I don't think either of us is getting anything useful out of this argument, so I'll stop now :)

Richard Brockie Aug 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!

Major Denis Bloodnok Aug 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.)

Richard Brockie Aug 16, 2025

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

Major Denis Bloodnok Aug 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.

Richard Brockie Aug 16, 2025

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

Major Denis Bloodnok Aug 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.

Richard Brockie Aug 17, 2025

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.

Major Denis Bloodnok Aug 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.

Major Denis Bloodnok Aug 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?

Andrew Drake Aug 12, 2025

@SludgePhD @danderson the total lifetime reported adjustment was "almost 0.5 picometers", or 50 thousandths of an Angstrom. That's only two digits of precision on the full value, hardly the territory of future-proofing by using unnecessarily fine-grained units.

I don't know how these particular drives adjust their fly height. One mechanism introduced by HGST in 2007 was a heating coil in the head to expand the tip slightly, causing it to project closer to the drive surface. Spitballing, if you control the current to the heating element with a DAC that has its maximum value set to a current that would expand the drive head tip by 10% the nominal fly height (0.1nm), a 14-bit DAC gives you <0.01pm adjustments. That level of precision is easy: even a cheap digital audio player has a 16-bit DAC. If I had to guess, the adjustment range of that system is much smaller than 10%, and so an even smaller DAC would be adequate.

Richard Brockie Aug 16, 2025

@adrake @SludgePhD @danderson

Yes, that’s still how it’s done

Major Denis Bloodnok Aug 12, 2025

@SludgePhD @danderson I rather think I've been blocked for pointing out this is hooey, but FWIW I grabbed Seagate's "openSeachest" software from git and grepped it for "pico", "metre", and "ngstr" - these strings don't appear. I presume this is just an error on the OP's part.