4 ms·

Well, uh, yes. No one really expects that rotational rust will get much faster, and in fact history shows that, compared to the increase in density, the increa

by dom0 10y ago

Well, uh, yes. No one really expects that rotational rust will get much faster, and in fact history shows that, compared to the increase in density, the increase in transfer rates are laughable at best. Between 1990 and today you are probably looking at a 20 000 times increase in density, yet transfer rates only increased around a factor of around 150-200. [In fact, from the early 1960 to today it's only a factor of about 1000. There are quite possibly few performance metrics that increased so slowly as disk transfer rate]. Why is that? Increasing density does only marginally increase transfer speed: Most density increases are achieved by packing more tracks onto the platter, while storing more sectors per track plays a minor role. But a single R/W head can only read a single track, not parallel tracks, hence speed only increases if you pack more sectors into each track, not by increasing the number of tracks. That's why between today and 10 years ago performance in desktop or server drives only differs a little, compared to the capacity increase to 8+ TB in a 3.5" drive. More platters also don't help in transfer rate, because the alignment of all heads on the actuator is fixed: at any given time only one platter and one R/W head is used (locked to the track). [More platters can help reduce seek time in certain scenarios though] More disks on the other hand...

kardos 10y ago
Is there a reason you couldn't use a second set of actuators?
- dom0 10y ago
  There's kinda not enough space: https://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Samsung_HD753LJ_03-Opened.jpg/1024px-Samsung_HD753LJ_03-Opened.jpg https://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Sa... You could probably do it if the platter diameter were reduced, which would then accordingly reduce capacity as well. At that point you could just as well use two drives and also get lower overall failure probability. Or you use SSD caches, or memory caches, ...
  - kardos 10y ago
    I was thinking of a Frankendrive with 5 actuators, which can do 5 parallel reads/writes, as long as they don't overlap. But the new dimensions would probably be the roadblock.
    - digler999 10y ago
      I was going to ask this too, but start with a simple 2nd voice coil on the opposite side of the spindle, keep the 3.5" form factor and just make it longer. My guess is the motion of one actuator would disrupt the airflow of the other, since we're talking about micrometer-ish (?) mechanical tolerances. This might be able to be mitigated by interleaving which side each arm wrote to. So the left arm's heads address the tops of the platters, the right side addresses the bottoms. Perhaps another option is to go back to the 1980's hdd designs where the arm moves straight down the radius of the platter. This design might permit multiple heads on the same arm. I'm sure all this stuff has been researched thoroughly. Either way, this doubles/triples the probability of mechanical failure
      - hga 10y ago
        Perhaps another option is to go back to the 1980's hdd designs where the arm moves straight down the radius of the platter. This design might permit multiple heads on the same arm. At my first job, sysadmin and programming on a PDP-11/44, 1-2 fascinating Winchester drives were procured for it. They had clear plastic covers, and you could see everything, the disk, the actuator, which had two heads, and it was by and large square, I'm almost positive it was rotary, not solenoid based straight stroke like many '70-80s drives, CDC's famous line in particular.
      - dom0 10y ago
        > since we're talking about micrometer-ish (?) mechanical tolerances. Even less than that. Tracks are 100-300 nanometers wide; the head flies about 3-6 nm above the platter. The actuator mechanism literally locks onto the signal encoded in the magnetic track and follows it as the platter rotates underneath the R/W head.
  - 10y ago
hinkley 10y ago
That's not entirely true. Higher density means more data per track, not just more tracks per disk. You get an entire track per revolution so a track with more data is more MBps. So linear reads on a higher density drive are faster, and semi-linear accesses (ie, reading two files that are next to each other) do get faster. I remember reading a story about a guy who built a drive array with high capacity 7200 RPM drives that got within 20% of the performance of the 10K RPM setup they had, by partitioning the drives at the same capacity as the 10K equivalent. The head only had half as many tracks to traverse, so worst case access time was better, and the higher density made up for the lower RPMs.
- baruch 10y ago
  Short stroking helps get more performance from the disk, at least in terms of latency. The bandwidth change occurs because the density is fixed and there are more blocks on the outer rings per track compared to the inner rings so in one revolution you can read more tracks and you don't need to change tracks so often. You parent comment is right though, there are only small changes in bit density on the track in recent years so the bandwidth is not improving by much.
  - hinkley 10y ago
    So we went from 1TB to 8TB disks just by packing 3x as many tracks onto drives? I find that hard to believe, and the benchmarks agree with me. You don't double the write throughout on a disk by doubling the number of tracks. You need more platters and/or sectors per track to do that.
    - deleted 10y ago
      [deleted]
- dom0 10y ago
  > Higher density means more data per track, not just more tracks per disk. I literally said that: > Most density increases are achieved by packing more tracks onto the platter, while storing more sectors per track plays a minor role.
mahyarm 10y ago
How about the stupid idea of making a minimum block size be a multiple of the number of platters to write to. The block is divided evenly amongst all platters and are always at the same parallel track/sector. That way you can multiply the read/write speeds to be a function of the number of platters. Instead of 50-100MB/s, you can get 4-8x the speed in large linear transfers, which helps get dead racks back up faster and would work quite well in backblaze's backup model. Your block sizes will be huge, but I think in backblaze's case that doesn't really matter so much.
- eye2sky 10y ago
  You cannot read from multiple platters in parallel. When you are aligned to read from a certain track on a certain side of a platter, you are not necessarily aligned on all platters. This head mechanism is not accurate enough for that.
  - deleted 10y ago
    [deleted]
- rzzzt 10y ago
  Google's paper on datacenter hard drives addresses this point -- it's easier to do this and other tricks with a set of disks than attempting it within the confines of a single drive. https://news.ycombinator.com/item?id=11172876 https://news.ycombinator.com/item?id=11172876
mapt 10y ago
Or more read-write heads?

by dom0 10y ago

kardos 10y ago

Is there a reason you couldn't use a second set of actuators?

dom0 10y ago
There's kinda not enough space: https://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Samsung_HD753LJ_03-Opened.jpg/1024px-Samsung_HD753LJ_03-Opened.jpg https://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Sa... You could probably do it if the platter diameter were reduced, which would then accordingly reduce capacity as well. At that point you could just as well use two drives and also get lower overall failure probability. Or you use SSD caches, or memory caches, ...
- kardos 10y ago
  I was thinking of a Frankendrive with 5 actuators, which can do 5 parallel reads/writes, as long as they don't overlap. But the new dimensions would probably be the roadblock.
  - digler999 10y ago
    I was going to ask this too, but start with a simple 2nd voice coil on the opposite side of the spindle, keep the 3.5" form factor and just make it longer. My guess is the motion of one actuator would disrupt the airflow of the other, since we're talking about micrometer-ish (?) mechanical tolerances. This might be able to be mitigated by interleaving which side each arm wrote to. So the left arm's heads address the tops of the platters, the right side addresses the bottoms. Perhaps another option is to go back to the 1980's hdd designs where the arm moves straight down the radius of the platter. This design might permit multiple heads on the same arm. I'm sure all this stuff has been researched thoroughly. Either way, this doubles/triples the probability of mechanical failure
    - hga 10y ago
      Perhaps another option is to go back to the 1980's hdd designs where the arm moves straight down the radius of the platter. This design might permit multiple heads on the same arm. At my first job, sysadmin and programming on a PDP-11/44, 1-2 fascinating Winchester drives were procured for it. They had clear plastic covers, and you could see everything, the disk, the actuator, which had two heads, and it was by and large square, I'm almost positive it was rotary, not solenoid based straight stroke like many '70-80s drives, CDC's famous line in particular.
    - dom0 10y ago
      > since we're talking about micrometer-ish (?) mechanical tolerances. Even less than that. Tracks are 100-300 nanometers wide; the head flies about 3-6 nm above the platter. The actuator mechanism literally locks onto the signal encoded in the magnetic track and follows it as the platter rotates underneath the R/W head.
- 10y ago

hinkley 10y ago

That's not entirely true. Higher density means more data per track, not just more tracks per disk. You get an entire track per revolution so a track with more data is more MBps. So linear reads on a higher density drive are faster, and semi-linear accesses (ie, reading two files that are next to each other) do get faster. I remember reading a story about a guy who built a drive array with high capacity 7200 RPM drives that got within 20% of the performance of the 10K RPM setup they had, by partitioning the drives at the same capacity as the 10K equivalent. The head only had half as many tracks to traverse, so worst case access time was better, and the higher density made up for the lower RPMs.

baruch 10y ago
Short stroking helps get more performance from the disk, at least in terms of latency. The bandwidth change occurs because the density is fixed and there are more blocks on the outer rings per track compared to the inner rings so in one revolution you can read more tracks and you don't need to change tracks so often. You parent comment is right though, there are only small changes in bit density on the track in recent years so the bandwidth is not improving by much.
- hinkley 10y ago
  So we went from 1TB to 8TB disks just by packing 3x as many tracks onto drives? I find that hard to believe, and the benchmarks agree with me. You don't double the write throughout on a disk by doubling the number of tracks. You need more platters and/or sectors per track to do that.
  - deleted 10y ago
    [deleted]
dom0 10y ago
> Higher density means more data per track, not just more tracks per disk. I literally said that: > Most density increases are achieved by packing more tracks onto the platter, while storing more sectors per track plays a minor role.

mahyarm 10y ago

How about the stupid idea of making a minimum block size be a multiple of the number of platters to write to. The block is divided evenly amongst all platters and are always at the same parallel track/sector. That way you can multiply the read/write speeds to be a function of the number of platters. Instead of 50-100MB/s, you can get 4-8x the speed in large linear transfers, which helps get dead racks back up faster and would work quite well in backblaze's backup model. Your block sizes will be huge, but I think in backblaze's case that doesn't really matter so much.

eye2sky 10y ago
You cannot read from multiple platters in parallel. When you are aligned to read from a certain track on a certain side of a platter, you are not necessarily aligned on all platters. This head mechanism is not accurate enough for that.
- deleted 10y ago
  [deleted]
rzzzt 10y ago
Google's paper on datacenter hard drives addresses this point -- it's easier to do this and other tricks with a set of disks than attempting it within the confines of a single drive. https://news.ycombinator.com/item?id=11172876 https://news.ycombinator.com/item?id=11172876

mapt 10y ago

Or more read-write heads?