6 ms·

They are adding multiple actuators, which actually should help that fundamental limitation.

by Robotbeat 6y ago

altcognito 6y ago
Could easily reduce reliability.
- Robotbeat 6y ago
  It’s possible, just as increasing the density of NAND with 4 Levels per cell reduces write count. But can be countered by more careful engineering of the device.
  - altcognito 6y ago
    NAND failures tend to be more localized though.
    - Robotbeat 6y ago
      This is probably not representative because it was using low grade USB flash devices from 10-15 years ago, but I think I’ve had more NAND flash failures than hard drive failures. I don’t think doubling the number of heads would make a huge difference as you wouldn’t be quite halving the reliability.
      - altcognito 6y ago
        Oh, I totally agree in that regard. Hard drive technology has been mature for a good number of decades, so a lot of the kinks have been worked out.
- jacquesm 6y ago
  No, it will reduce reliability. More parts = worse reliability.
  - Robotbeat 6y ago
    Maybe? The issue is mechanical devices can be engineered until the reliability level needed is achieved. Sure, take the same device and double the numbers, you’ll have half the reliability, but you’ll also have more parts being made and more parts to amortize the reliability improvement engineering over (and more parts to get reliability statistics from... which also means you can test the rest of the hard drive faster, which can provide other reliability enhancements). NAND devices have similar constraints, actually, but they’re reaching more fundamental limits due to quantum effects and they’re generally addressed with software/firmware tricks like wear leveling.
    - jacquesm 6y ago
      > The issue is mechanical devices can be engineered until the reliability level needed is achieved. And if wishes were horses... No, you can not simply state some arbitrary level of reliability and design to that level. At any given level of reliability more parts = less reliability and more time or resources spent on design are not by going to remediate that unless you are willing to accept much higher costs. Engineering is a trade-off and physics determine the sweet spot for the balance of that trade off. Once that sweet spot has been determined all other things being equal more parts will cause your reliability to go down unless you will also accept that your costs will go up and/or other parameters will be affected in a negative way. Software people in general have a very hard time to understand this because to them 'parts' are free, but even in software, assuming zero costs for parts (adding a library, a function, a line of code) has an effect on reliability. That's why computers used to be much more reliable than they are today, and that's before we get into details such as cognitive load while trying to understand complex systems.
rzzzt 6y ago
Where do those fit, in the opposite corner?
notacoward 6y ago
Not much, actually. Dual actuators only improve parallelism, not media transfer rate or latency. In practice, dual actuators don't even double external transfer rates due to occasional latency (which is unavoidable even for the best-case scenarios) and internal contention elsewhere in the drive. Even if you had four actuators working perfectly in concert, and weren't bottlenecked on the external interface speed (which you would be), a 4x improvement in transfer rate vs. a 8x increase in capacity would still mean a 2x increase in fill/empty time. As with the shift from 5.25" to 3.5" to 2.5" and even 1.8" drives, the only way out of this bind is really to take advantage of the improved areal density to make drives that are the same capacity but smaller and pack more of those into the same volume or power/heat envelope. Drive manufacturers could help this along a little bit e.g. by sharing a motor and some environmental bits between what are otherwise completely separate drives (including separate external interfaces) within a single package, but mostly we'd all better get used to higher drive counts. Dual actuators - and this is far from the first time they've been tried BTW - are mostly a red herring. Background: I worked on exactly these problems for the latter half of a thirty-year career, most relevantly at my last job working on an exabyte-scale storage system at a FAANG.
- effie 6y ago
  Interesting. What do you think about Seagate's claim that dual actuator decreases their costs because it takes less time to test the drive? Is that the real reason for dual actuator?
- PaulHoule 6y ago
  Also what market is there for big hard drives outside of FAANG and a few other places? Retailers in my area (e.g. Best Buy) don't stock hard drives larger than 4 TB; I was going to tell somebody who lived in the Valley that he's lucky to be able to go to Fry's and then Fry's closed down. Those same retailers stock both budget and quality SSD's up to 2TB in size. Most upgraders and the system builders are happy. For the rest of us there is Amazon where the Seagate Exos "enterprise" drive costs half of what similar "consumer" drives cost, has a great reputation and does not seem hard to live with at home. I would not take it for granted at all that a backup, RAID rebuild, restore, metadata scan or any full scan would work on such a disk if I hadn't tested it -- it is just that kind of technology. Synology is not crazy at all when they make you buy branded large drives to go in the enclosure.

They are adding multiple actuators, which actually should help that fundamental limitation.

altcognito 6y ago

Could easily reduce reliability.

Robotbeat 6y ago
It’s possible, just as increasing the density of NAND with 4 Levels per cell reduces write count. But can be countered by more careful engineering of the device.
- altcognito 6y ago
  NAND failures tend to be more localized though.
  - Robotbeat 6y ago
    This is probably not representative because it was using low grade USB flash devices from 10-15 years ago, but I think I’ve had more NAND flash failures than hard drive failures. I don’t think doubling the number of heads would make a huge difference as you wouldn’t be quite halving the reliability.
    - altcognito 6y ago
      Oh, I totally agree in that regard. Hard drive technology has been mature for a good number of decades, so a lot of the kinks have been worked out.
jacquesm 6y ago
No, it will reduce reliability. More parts = worse reliability.
- Robotbeat 6y ago
  Maybe? The issue is mechanical devices can be engineered until the reliability level needed is achieved. Sure, take the same device and double the numbers, you’ll have half the reliability, but you’ll also have more parts being made and more parts to amortize the reliability improvement engineering over (and more parts to get reliability statistics from... which also means you can test the rest of the hard drive faster, which can provide other reliability enhancements). NAND devices have similar constraints, actually, but they’re reaching more fundamental limits due to quantum effects and they’re generally addressed with software/firmware tricks like wear leveling.
  - jacquesm 6y ago
    > The issue is mechanical devices can be engineered until the reliability level needed is achieved. And if wishes were horses... No, you can not simply state some arbitrary level of reliability and design to that level. At any given level of reliability more parts = less reliability and more time or resources spent on design are not by going to remediate that unless you are willing to accept much higher costs. Engineering is a trade-off and physics determine the sweet spot for the balance of that trade off. Once that sweet spot has been determined all other things being equal more parts will cause your reliability to go down unless you will also accept that your costs will go up and/or other parameters will be affected in a negative way. Software people in general have a very hard time to understand this because to them 'parts' are free, but even in software, assuming zero costs for parts (adding a library, a function, a line of code) has an effect on reliability. That's why computers used to be much more reliable than they are today, and that's before we get into details such as cognitive load while trying to understand complex systems.

rzzzt 6y ago

Where do those fit, in the opposite corner?

notacoward 6y ago

Not much, actually. Dual actuators only improve parallelism, not media transfer rate or latency. In practice, dual actuators don't even double external transfer rates due to occasional latency (which is unavoidable even for the best-case scenarios) and internal contention elsewhere in the drive. Even if you had four actuators working perfectly in concert, and weren't bottlenecked on the external interface speed (which you would be), a 4x improvement in transfer rate vs. a 8x increase in capacity would still mean a 2x increase in fill/empty time. As with the shift from 5.25" to 3.5" to 2.5" and even 1.8" drives, the only way out of this bind is really to take advantage of the improved areal density to make drives that are the same capacity but smaller and pack more of those into the same volume or power/heat envelope. Drive manufacturers could help this along a little bit e.g. by sharing a motor and some environmental bits between what are otherwise completely separate drives (including separate external interfaces) within a single package, but mostly we'd all better get used to higher drive counts. Dual actuators - and this is far from the first time they've been tried BTW - are mostly a red herring. Background: I worked on exactly these problems for the latter half of a thirty-year career, most relevantly at my last job working on an exabyte-scale storage system at a FAANG.

effie 6y ago
Interesting. What do you think about Seagate's claim that dual actuator decreases their costs because it takes less time to test the drive? Is that the real reason for dual actuator?
PaulHoule 6y ago
Also what market is there for big hard drives outside of FAANG and a few other places? Retailers in my area (e.g. Best Buy) don't stock hard drives larger than 4 TB; I was going to tell somebody who lived in the Valley that he's lucky to be able to go to Fry's and then Fry's closed down. Those same retailers stock both budget and quality SSD's up to 2TB in size. Most upgraders and the system builders are happy. For the rest of us there is Amazon where the Seagate Exos "enterprise" drive costs half of what similar "consumer" drives cost, has a great reputation and does not seem hard to live with at home. I would not take it for granted at all that a backup, RAID rebuild, restore, metadata scan or any full scan would work on such a disk if I hadn't tested it -- it is just that kind of technology. Synology is not crazy at all when they make you buy branded large drives to go in the enclosure.