[haasn]

I think a better way to look at it is not the chance of total data loss, but the expected running cost over time. Even with just RAID6 it's hard to imagine a scenario in which you would be unable to replace one of the failed drives and resilver before getting two further failures, so the naive reliability calculations are extremely biased.

So if we assume that we can safely operate at some fixed level of redundancy (say 3 copies) across all hard drive vendors, the only question is (how many drives you need to replace per year) × (the price of those drives).

[Veratyr]

The problem for me at least is that most of the drives I use come with 3ish year warranties, so they're RMA'ed for free. By the time I'd have to buy a replacement I'd be intending to upgrade anyway.

[aidenn0]

RAID6 is unusable for many types of "always on" data, in particular databases backing high volume sites; the performance drops so much during a rebuild that for many uses it might as well be offline.

[throwaway7767]

Then you use mirrors, you can apply a similar calculation there. Lower price per disk means you can put more disks in each mirror and compensate for lower per-disk reliability.

Obviously the type of redundancy used will depend on your specific application requirements, including performance.