Comment by cantrevealname

I think it’s a mistake to test the worn and fresh disks at different intervals. I.e., testing worn disks in years 1 and 3, and fresh disks in years 2 and 4.

Let’s say that the worn disks are found to have failed the hash check in year 1 and the fresh disks are found to have failed in year 2. Can you conclude that worn and fresh are equally bad? No, you can’t, because maybe the fresh disks were still OK in year 1 — but you didn’t check them in year 1.

As another example, suppose the worn disks are found to be good in year 1 but the fresh disks are found to be bad in year 2. This seems like an unlikely result, but if it happened, what could you conclude? Well, you can’t conclude anything. Maybe worn is better because they are still good in year 2, but you aren’t checking them in year 2. Maybe fresh is better because the worn will fail in year 1.1 but the fresh last until year 1.9 before failing. Maybe all the disks fail in year 1.5 so they are equally bad.

I think it’s better to test the disks at the same intervals since you can always draw a conclusion.