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...
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, ...
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
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.
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.
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.
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.
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.
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.
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.
Guess you are not throwing those away so what are you doing with them.
https://www.backblaze.com/blog/hard-drive-reliability-q4-201...
That takes ~11 hours to fill. At 400x the density it would take 20x as long or 9 days. I don't think HDD drives are hitting 400x the density any time soon but if they did it would be a problem.
However, in an array you could take a month to fill a drive to 75% without causing to much trouble. Assuming you had enough drives. That's around a ~80PB limit drive. IMO, the real issue is it would take another month to download all that data. Relegating HDD firmly into archival storage.
PS: I don't think rust is going to get into those density's making this far less of an issue.
The smart move is to keep several smaller arrays instead of one big one. This lowers risk as well. I dont put in anything bigger than 7 disks into production. Past that I'm just asking for trouble. Its better to have 4 7 disk arrays than one 28 disk array. A drive fail means a quick rebuild and a restore is going to be 1/4 the time.