I run a RAID10 utilizing 4x 4TB disks, created with mdadm and without a dedicated raid controller (four hard drives plugged right into the server's motherboard). When it was originally set up it utilized disks by partition numbers - /dev/sda1, /dev/sdb1, etc. Well I had a disk fail (/dev/sdd) and just swapped it out for a fresh one, and when I added it to the array I accidentally added it as /dev/sdd instead of /dev/sdd1. It's currently rebuilding and will be for a few more hours, but it doesn't seem to have caused any problems yet.
So it got me wondering, for a basic RAID without any particular special configuration, what's the difference between a raid comprised of partitions (one partition per disk) and a raid of the devices themselves? And while I'm on the topic, will my new RAID disk cause any trouble since it was added as a device instead of as a partition?
[root@fluttershy ~]# mdadm --detail /dev/md0 /dev/md0: Version : 1.1 Creation Time : Wed Feb 19 05:43:49 2014 Raid Level : raid10 Array Size : 7813770240 (7451.79 GiB 8001.30 GB) Used Dev Size : 3906885120 (3725.90 GiB 4000.65 GB) Raid Devices : 4 Total Devices : 4 Persistence : Superblock is persistent Intent Bitmap : Internal Update Time : Sun Nov 8 20:32:28 2015 State : clean, degraded, recovering Active Devices : 3 Working Devices : 4 Failed Devices : 0 Spare Devices : 1 Layout : near=2 Chunk Size : 512K Rebuild Status : 3% complete Name : fluttershy:0 (local to host fluttershy) UUID : 45f8673c:332503c8:a9307bae:4177f753 Events : 115588 Number Major Minor RaidDevice State 0 8 1 0 active sync set-A /dev/sda1 1 8 17 1 active sync set-B /dev/sdb1 2 8 33 2 active sync set-A /dev/sdc1 4 8 48 3 spare rebuilding /dev/sdd [root@fluttershy ~]#