# Why doesn't hard disk performance scale linearly with capacity

If I take a 500GB hard drive and a 1TB hard disk otherwise identical, why is the 1TB drive not twice as fast as the 500GB drive?

If they are both spinning at the at the same rate, and the the 1TB drive has twice the data density (as it must), it should have a transfer rate of twice as much (although seek times will be comparable) or am I missing something.

-
Some useful info has been posted here (the antithesis of this question): superuser.com/questions/21486/are-smaller-hard-drives-faster –  sblair Nov 7 '09 at 2:34
That’s like asking why a train in Tokyo cannot transfer as many people as a train in Connecticut (assuming relatively equal train and station sizes and train speeds). –  Synetech Dec 30 '13 at 18:41

If they are both spinning at the at the same rate, and the the 1TB drive has twice the data density (as it must), it should have a transfer rate of twice as much

Actually, the capacity is proportional to the area, whereas the transfer rate is proportional to the radius. Assuming that the number of tracks increases in proportional with the amount of data per track, the transfer rate should increase with the square root of the increase in capacity.

So increasing from 500GB to 1000GB you double the density per area of the drive, but the density per track is only increased by a factor of 1.414 (the square root of 2), so the transfer rate is only increased by a factor of 1.414.

I suspect the real factor may be a bit higher because it may be easier to increase the density per track than the number of tracks.

-
Good insights, however usually when a vendor releases a series of drives, the capacity is dictated by the number of platters. However this could be a factor when comparing drives from different manufacturers. –  Alister Jan 7 at 5:36
@Alister - yes, the capacity is also dictated by the number of platters, but you specifically mentioned data DENSITY. The area increases with the number of platters, not the density. So if you change the capacity purely by changing the number of platters, you contradict your own "the 1TB drive has twice the data density (as it must)" claim - the density of information per unit area (the measure normally quoted for a technology) is precisely the same. –  Steve314 Jan 7 at 7:55
@Alister - anyway, if you expect multiple platters to be read at once, I'm sure that's not the case. The drive heads have to follow little deviations in track position to access the data. These deviations vary over time (due to temperature etc) and from surface to surface. So I don't think it's practical to read multiple surfaces at the same time - doing so would need a much more complex and expensive mechanism - though I don't really know. You do gain, though, by needing fewer seeks as more data is in the same cylinder of tracks - a seek isn't the same as just tracking deviations in a track. –  Steve314 Jan 7 at 8:10
@Alister - actually, I think early hard disks did read all surfaces at the same time, but that was a long time ago. Possibly so far back that they were still called Winchester disks. In a way, it's a bit like the parallel vs. serial cable fallacy - naively you'd think more wires means faster, but with wire lengths and temperatures varying slightly, capacitance and crosstalk between the wires etc, you get practical issues that increase costs and/or slow things down. –  Steve314 Jan 7 at 8:22