Speed. Why didn't you use RAID 6? Surely that makes more sense; the more disks you have in array the more likely it is that one of them is likely to fail, with RAID 5 you can lose no more than 1 disk before all data is lost, at least with RAID6 you get the benefits of 2 disk loss. Anyway yeah, my plan is to create 3 sets of 15 disks, I suspect speed will be an issue at the physical level, this is even before I introduce a filesystem, so the FS needs to be absolutely optimal with regards to speed.

RAID 6 is slower than RAID 5. We do have "hot spare", so when a drive fails (we've had one fail in the 8 months we've had 6 of these arrays) it will automatically rebuild. The only risk is having a 2nd fail during the rebuild.

Of course if I want more speed, I'd do RAID 10. But then I'd get less space. That's the usual tradeoff.

Maybe I'll get some quality time, before the deployment deadline, with the next array we get to do some real benchmarks of the various choices (RAID 5 vs 6 vs 10 vs 50 vs 60 ... and EXT4 vs XFS vs ZFS).

With 45TB of space you will most likely have to deal with faulty disks a lot, becuase I know this is some type of RAID we are dealing with here.