My production server is on 1.7.5 and I expect it to remain that way for quite a while, although it looks to me like even for the 2.x series the guidance is "use 1 for spinning disk!" with no clear distinction between drives and arrays.
If I used the raw algorithm documented there it'd end up recommending I that allocate 4 threads (min(4,(24/2))... it'd be around 300 MB/s/thread at that point. However, my understanding is that there is a diminishing return on concurrency for RAID writes on spinning disk, so I'm wondering if I should drive a little lower in that case. (Edit: first time I read it as if the calculation would be 12 threads; 4 seems almost sane).
I guess I'm wondering if the guidance is "for anything at all that spins" or if it has more to do with I/O throughput and so RAID would be a potential exemption, even if with some hesitation.