I'd not be surprised if they since switched to using a true spiral.
And the head count is one assembly per disk-surface for almost all devices. So 2 per disk for HDDs.
Yes, reallocated sectors aren't accounted for, but they should be so rare as to not matter outside of hard realtime applications, which shouldn't be using such corner-cutting devices anyways.
That spiral assumption is due to the servo tracks they need and that the inter-sector gap only has to be sized to account for how fast they can switch the write head from idle to spewing bits, so they have incentive to make it smaller than what they'd likely want the head to be able to jump when continuously streaming data.
Multiple platters would likely just mean that random writes with suitable alignment cost the same until you reach the effective platter count, as the sectors that fly by at the same time should be sequential.
SSDs on the other hand use some complicated LSM trees or similar datastructures.
Yes, reallocated sectors aren't accounted for, but they should be so rare as to not matter outside of hard realtime applications, which shouldn't be using such corner-cutting devices anyways.
That spiral assumption is due to the servo tracks they need and that the inter-sector gap only has to be sized to account for how fast they can switch the write head from idle to spewing bits, so they have incentive to make it smaller than what they'd likely want the head to be able to jump when continuously streaming data.
Multiple platters would likely just mean that random writes with suitable alignment cost the same until you reach the effective platter count, as the sectors that fly by at the same time should be sequential.
SSDs on the other hand use some complicated LSM trees or similar datastructures.