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Comment by bastawhiz | Hacker News Reader

They keep saying it doubles the iops, but is that really accurate? From the diagram it seems like half of the platters have their own actuator. Surely unless the data is physically distributed across the platters in a really funky way you'd only see a doubling of iops when accessing two different pieces of data that are each found on platters addressed by their own actuators.

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I was thinking that the controller probably takes care of interleaving data across platters, so at least small enough random I/Os would be able to use only one actuator.

I did some more reading and the PDF [1] says it shows up as two separate LUNs behind a single SAS port, but sharing the drive cache.

But what I initially didn't catch (and another comment here pointed out), is how come they claim to do over 500 MB/s of sequential scanning I/O. Small random IOPS I get, but for large sequential I/Os you still have the same amount of platters, with the same bit-density, rotating at the same speed... Or is even the sequential I/O throughput "bottleneck" mostly dominated by the tiny seeks + calibration to the next adjacent track and not the actual magnetic reading part?

I now really want to buy one for (some?) reason. Could get an used version from amazon for $150 but I'll try to hold back. It's also interesting that there are no new, unused versions available there (discontinued?)

[1] https://www.seagate.com/files/www-content/solutions/mach-2-m...

Transfer rates can be doubled by spreading (something like) RAID 0 across those two LUNs, if doubling transfer rate and treating it all as one logical device is the goal.

The drive doesn't know how to do this by itself.

(That doesn't mean that the marketing wank is a lie: It's still a singular drive with a singular SKU. The marketing wank may easily be considered to be incomplete, however.)

These drives have been available for a fair time now, though they're still fairly new. They were never intended for plebs like us to be able to buy from places like Amazon, probably because the support costs would be through the roof for such an unusual product with such niche use-cases.

They probably stripe the data across the banks of platters just like multiple drives in raid 0, so when reading or writing a sufficiently large file, both banks are always operating at full speed.

But if the number of platters, rotation speed and bit-density is exactly the same as in regular drives, the number of “bits” moving past the read/write heads per rotation would still stay the same? Would be interesting to know how much time typically is spent on actually reading the bits vs the tiny seeks & track calibration when reading, say, 10 GB sequentially.

Edit: other comments below explain the missing part, in a regular drive, apparently just one head can read/write (to one platter) at a time due to calibration reasons, even if your disk physically has 8 platters. So having two actuators allows two platters out of 8 to do IO concurrently.

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