There might be surprisingly little value in going tape due to all the specialization required. As the other comment suggest, many of the lower tiers likely represent basically IO bandwidth classes. a 16 TB disk with 100 IOPs can only offer 1 IOP/s over 1.6 TB for 100 customers, or 0.1 IOP/s over 160 GB for 1000, etc. Just scale up that thinking to a building full of disks, it still applies
I realize you're making a general point about space/IO ratios and the below is orthogonal, no contradiction.
It's actually a lot less user-facing per disk IO capacity that you will be able to "sell" in a large distributed storage system. There's constant maintenance churn to keep data available:
- local hardware failure
- planned larger scale maintenance
- transient, unplanned larger scale failures
(etc)
In general, you can fall back to using reconstruction from the erasure codes for serving during degradation. But that's a) enormously expensive in IO and CPU and b) you carry higher availability and/or durability risk because you lost redundancy.
Additionally, it may make sense to rebalance where data lives for optimal read throughput (and other performance reasons).
So in practice, there's constant rebalancing going on in a sophisticated distributed storage system that takes a good chunk of your HDD IOPS.
This + garbage collection also makes tape really unattractive for all but very static archives.
See comments above about AWS per-request cost - if your customers want higher performance, they'll pay enough to let AWS waste some of that space and earn a profit on it.
