6 ms·

When I first heard about how mechanical hard drives work, back in the days when the highest end 300 MB (yes MB) drives ran a stack of four or six platters, and

When I first heard about how mechanical hard drives work, back in the days when the highest end 300 MB (yes MB) drives ran a stack of four or six platters, and some drives still used a sort of rack and pinion positioning setup rather than a voice coil, I assumed each platter would have its own actuator and heads were only paired up top and bottom per platter. I was surprised to find that instead, the entire rack of them moved as a single unit, with so much energy wasted overcoming 3 or 4 times the necessary inertial mass for each seek. So now, 20 something years later, well after SSDs have come close to superceding spinning rust when all tradeoffs and variables are considered together, someone has finally gotten around to putting an extra actuator in these units. Great! While on that subject I also never understood why the general layout of components inside these drives never iterated away from the very earliest RLL / MFM winchester mechanisms, with the spinning platter assembly slightly off-center in the case, and a single head assembly off in one corner. I assumed at some point that the armature assembly would become miniaturized just enough to allow centering the platter spindle and adding another set of heads at the opposite corner of the case, so that each head set could address opposite sides of the platter, effectively halving access times even on single platter drives.

HPsquared 2y ago
It might be difficult to control the heads if each one has other heads moving right next to it. Magnetic effects of the voice coils, mechanical vibration, maybe electrical even. It seems a much harder control problem given that the requirements are so stringent.
- jimbooonooo 2y ago
  Rejection of neighboring HDD vibe disturbances is already an issue, so fascinating to see what their control approach is. In my limited experience, mechanical HDD control is near top of the game for electromechanical positioning.
  - HPsquared 2y ago
    At least they know what all the heads are doing so could design them to cooperate or model/predict the disturbances pretty well, unlike for external vibrations from neighbouring disks.
doe_eyes 2y ago
The heads were very lightweight, and defragmentation algorithms were supposed to make sure that the heads seldom had to move far. Far more energy was wasted spinning the platters. And the benefit of the old system was that you only needed one system for ultra-precise positioning of the heads. This was probably not just simpler and more compact, but also more reliable. In a multi-actuator design, the failure of any of the independent positioning mechanisms kills the drive. Not saying that this isn't progress, but 3.5" and 2.5" HDDs were a true marvel of engineering when they came out, and maybe we needed a couple of breakthroughs in electromechanics to get to MACH.2.
- jimbooonooo 2y ago
  Agreed re: mechanical HDDs being engineering marvels. The positioning performance at the price/manf volumes is really incredible. As the poster below mentions, neighboring actuator coupling/disturbance rejection is very much a performance consideration (even between neighboring HDDs) and then to package all that additional complexity into the same form factor is really something. It'll be interesting to see if this sticks around, or the added complexity makes it short lived.
jiggawatts 2y ago
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!
- ChainOfFools 2y ago
  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.
Dalewyn 2y ago
> well after SSDs have come close to superceding spinning rust when all tradeoffs and variables are considered together SSDs are great, but I wouldn't put them that far. There are definite drawbacks to SSDs due to the nature of their design that HDDs don't have, and vice versa. Use the right tool for the job, as they say. Personally, I have both SSDs and HDDs in my rigs because I have workloads appropriate for either of them at a given time.
- aftbit 2y ago
  I also have both, but only because I can get HDDs for $10 to $20 per TB while SSDs still cost $50 to $100 per TB.
- quaintdev 2y ago
  Could you give examples of suitable workloads for HDD and SSD
  - Arn_Thor 2y ago
    I’m not the person you answered but SSD: boot disk, cache, video files you’re working off of. HDD: data you don’t mind waiting for such as video and audio storage, backups, or just cases where you have a lot of data and the SSD isn’t yet cost effective enough
  - Dalewyn 2y ago
    Workloads that require low latency, fast random access, don't involve too much (re)writes, mobile environment, money is not an object: SSDs. Workloads that involve lots of writes and especially rewrites, transient and ephemeral data, don't require low latency or fast random access, money is a factor: HDDs. The key thing is that HDDs have a theoretically infinite amount of writes compared to SSDs which have a finite amount. Certain workloads are consequently going to be more suited for HDDs.

HPsquared 2y ago

It might be difficult to control the heads if each one has other heads moving right next to it. Magnetic effects of the voice coils, mechanical vibration, maybe electrical even. It seems a much harder control problem given that the requirements are so stringent.

jimbooonooo 2y ago
Rejection of neighboring HDD vibe disturbances is already an issue, so fascinating to see what their control approach is. In my limited experience, mechanical HDD control is near top of the game for electromechanical positioning.
- HPsquared 2y ago
  At least they know what all the heads are doing so could design them to cooperate or model/predict the disturbances pretty well, unlike for external vibrations from neighbouring disks.

doe_eyes 2y ago

The heads were very lightweight, and defragmentation algorithms were supposed to make sure that the heads seldom had to move far. Far more energy was wasted spinning the platters. And the benefit of the old system was that you only needed one system for ultra-precise positioning of the heads. This was probably not just simpler and more compact, but also more reliable. In a multi-actuator design, the failure of any of the independent positioning mechanisms kills the drive. Not saying that this isn't progress, but 3.5" and 2.5" HDDs were a true marvel of engineering when they came out, and maybe we needed a couple of breakthroughs in electromechanics to get to MACH.2.

jimbooonooo 2y ago
Agreed re: mechanical HDDs being engineering marvels. The positioning performance at the price/manf volumes is really incredible. As the poster below mentions, neighboring actuator coupling/disturbance rejection is very much a performance consideration (even between neighboring HDDs) and then to package all that additional complexity into the same form factor is really something. It'll be interesting to see if this sticks around, or the added complexity makes it short lived.

jiggawatts 2y ago

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!

ChainOfFools 2y ago
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.

Dalewyn 2y ago

> well after SSDs have come close to superceding spinning rust when all tradeoffs and variables are considered together SSDs are great, but I wouldn't put them that far. There are definite drawbacks to SSDs due to the nature of their design that HDDs don't have, and vice versa. Use the right tool for the job, as they say. Personally, I have both SSDs and HDDs in my rigs because I have workloads appropriate for either of them at a given time.

aftbit 2y ago
I also have both, but only because I can get HDDs for $10 to $20 per TB while SSDs still cost $50 to $100 per TB.
quaintdev 2y ago
Could you give examples of suitable workloads for HDD and SSD
- Arn_Thor 2y ago
  I’m not the person you answered but SSD: boot disk, cache, video files you’re working off of. HDD: data you don’t mind waiting for such as video and audio storage, backups, or just cases where you have a lot of data and the SSD isn’t yet cost effective enough
- Dalewyn 2y ago
  Workloads that require low latency, fast random access, don't involve too much (re)writes, mobile environment, money is not an object: SSDs. Workloads that involve lots of writes and especially rewrites, transient and ephemeral data, don't require low latency or fast random access, money is a factor: HDDs. The key thing is that HDDs have a theoretically infinite amount of writes compared to SSDs which have a finite amount. Certain workloads are consequently going to be more suited for HDDs.