Reminder: Have backups. Imagine losing 32TB of data in one go.
On a related note, remirroring or rebuilding a 32TB RAID drive would probably take days.
Local drives are more fast cache for data that should be stored reliably elsewhere (maybe hot, maybe cold, depending on provider and cost concerns).
1. https://www.servethehome.com/discussing-low-wd-red-pro-nas-h...
The practical issues of RAID rebuilds also don't quite work that way - the "drives die while rebuilding problem" has two components:
(1) is that beyond a certain size, the risk of bit-error while resilvering becomes a certainty due to all the data you're reading - ZFS fixes this problem with it's checksums basically.
(2) If all your disks were installed at the same time, then the chance of another disk failure is high because they're likely all in the same part of the mean-time-to-failure distribution.
(3) the risk of an error is higher because you're doing a lot more activity then normal, and so the additional stress is increasing the chance of triggering a failure.
Though if you want specific numbers, what would you estimate as the per-drive odds of 8TB and 40TB drives dying during a resilver? Let's say they take 2 and 7 days respectively.
But I would also expect these drives to be cheaper per TB, at least by the time HAMR is 2-3 generations old.
If you only do 5+3 to keep the same 160T available, it's 256TB or 128% paid for and the same 100% usable, so an even steeper 22% discount to break even.
And nobody that's price conscious would make a 5+3 RAID so I'm not very worried about such an extreme scenario.