If you’re planning on using CentOS 7 with mdadm RAID, please do avoid.
I never did identify the specific problem but it’s as buggy as a 10-day-old corpse.
Had ridiculous and unsolvable problems with this config creating a simple RAID 5 set with four drives. Should be very easy. Instead I’ve wasted loads of time on it.
Same drives, same config, same hardware — only difference was Ubuntu vs. CentOS.
Worked fine, no issues. Not one of the problems I had with CentOS and their version of mdadm.
CentOS and RAID — just don’t do it. (And avoid systemd in the process.)