Comment by jiggawatts | Hacker News Reader

Comment by jiggawatts | Hacker News Reader

Parent Full thread

For mechanical drives, the ratio between IOPS and capacity has been getting exponentially worse over time.[1] This means that random seeks available per unit of data is getting so bad now that in cloud hosting they consider a 128 KB read to be "one" operation for cost calculations. The capacity unit for a single I/O used to be 512 bytes!

This is why I came here to make the same comment you just did.

A modern 30 TB data centre drive has about 300 IOPS, so that's just one random seek per 100 GB per second! Ouch.

I don't get why manufacturers don't make drives with actuator arms in all four corners, and heads that can move independently on both sides of every platter.

There might be some vibration and cross-talk issues, but surely they can be overcome with modern digital servo control technology.

That would allow 4x the IOPS just form having four sets of arms, and then 2x the IOPS because there's arms above and below platters, and then Nx where 'N' is the number of platters. Let's say there's 10, so that's 4x2x10 = 80x the IOPS for the same capacity!

Another thing I was always curious about is why swing-arm actuators stuck around all these years, rather than moving to a single fixed rail with a strip of individually addressable read regions, and another rail separate (and upstream) from this with write capability).

No more mechanical actuation at all, eliminating a huge amount of complicated precision machined componentry as well as the voice coil itself.

Mounting this fixed rail to multiple structural points inside the case means there would be zero possibility of a head crash.

No cross interference or mode switching between reading and writing. The entire region passing under the rail can be scanned simultaneously, or if it's not possible to manufacture such a sensor at a data density to match the drive platter then perhaps the rail could shift a millimetre or so back and forth to allow micron-fine positioning from a milimeter-course array of heads. Much less mass to move, likely simplifies the math needed to meet the data in flight, and reduces the total reciprocating moment to a single linear axis solution.

I know I can't be the only person, nor the 1st, to have considered this idea. I suspect multiple variations may be lurking in the patent portfolios of the big storage mfrs. But I would love to know why nothing resembling it has ever been tried in production over the 30 or 40 something years that hard drives have been mass commoditized.

> perhaps the rail could shift a millimetre or so back and forth to allow micron-fine positioning from a milimeter-course array of heads.

You're talking in terms of millimeters and microns. That is roughly the scale that CDs operated at.

According to https://blog.stuffedcow.net/2019/09/hard-disk-geometry-micro...

> In the newest drive I have, average track pitch is 80 nm and an average bit is 17 nm in length.

That's for hard drives providing 1TB per (double-sided) platter, less than half the density of today's most advanced hard drives.

A relative of mine designed ink-jet printer heads as wide as the paper.

Something similar could work with hard drives: an array of about 3 to 4 thin strips of silicon chips with a line of read/write heads.

They could also be staggered in tiers, one row offset from the other.

I feel like there's an alternate universe where this is exactly the design that was adopted and someone is out there suggesting our universe's setup instead, asking why not make a single read-write head that gets whipped back and forth around at mind numbing speeds seeking to different spots on the surface of the platter, and everyone's pointing out how absurdly bad and pointless an idea that would be: think about how delicate such a setup is, you want to put a flying bit of metal over a rotating metallic surface at just a microscopic distance apart - you want to rely on the Bernoulli effect to pull it into just the right proximity to the surface, and this has to work in every possible temperature, vibration, and moisture and air pressure scenario? how do you compensate for inevitable wear and tear on the mechanism in the drift and loss of precision that will induce, what about all of the fine particulates that may develop from friction over time, how do you get lubricants to last so long, all the possibilities for terminal head crashes, how slow mechanical mechanisms are versus solid state, the comparatively massive additional power draw and heat this would introduce, and so on.

Without knowing anything about the specifics of material engineering whatsoever in this domain, my assumption is that Manufacturing the read-write heads maybe one of the most expensive technical challenges and perhaps a disproportionately large factor in the cost of the entire assembly, so perhaps asking for an entire strip of these things might be like ignorantly asking, if the light-refracting feature of a diamond is the most salient feature of a diamond ring, why not make a rings with diamonds all the way around instead of just one.

You're technically correct but I think rather missing the point. I got the scale wrong (guessed at the tolerances involved rather than looking them up), but obviously resolving a mechanical position to meet the actual tolerances is already a solved problem by current voice coil systems. A simplified version of the same mechanism could be applied to the line-of-heads solution I'm suggesting, considering that in both cases a comparable reciprocal mass is being moved back and forth, but in the scenario I'm proposing it would only need to be moved a millimeter (or two, or one, or a micron - all are far smaller of a movement than the comparatively huge arc swept by current head designs)

Whatever subdivisions within that bounding range are necessary to achieve equivalent (or superior) accuracy to flying head designs has already been proven to work in spite of the current designs' much more complicated and delicate arrangement.

Another way to look at is that the ratio between capacity and physical IOPS constraints have gotten exponentially better over time. It's just that you can't treat all that capacity as hot capacity anymore.

I recall a whitepaper on HP high-end storage arrays from over 10 years ago, their arrays already read and cached at least 256kB of data, even if you wanted only a 4kB block.

It's this just a response to the narrowing of the use cases for mechanical drives, so they specialize at long term, infrequent access, large storage?

Reply on news.ycombinator.com