What is the meaning of the 'Personalities' feature under /proc/mdstat

On some systems I see this :

Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty]
md1 : active raid1 sdb1[1] sda1[0]
      10485696 blocks [2/2] [UU]

md2 : active raid1 sdb2[1] sda2[0]
      477371328 blocks [2/2] [UU]

And other systems show :

Personalities : [raid1]
md0 : active raid1 sdb2[1] sda2[0]
      204788 blocks super 1.0 [2/2] [UU]

md1 : active raid1 sdb1[1] sda1[0]
      4193272 blocks super 1.1 [2/2] [UU]

md2 : active raid1 sda3[0] sdb3[1]
      483985276 blocks super 1.1 [2/2] [UU]
      bitmap: 0/4 pages [0KB], 65536KB chunk

I wonder what is the meaning of Personalities and the impact of having different values.

Solution 1:

The personalities are the RAID Levels that are supported by your Kernel. Your first example includes the [faulty] personality which can be used for diagnostic purposes. In Your second example the kernel (currently) only supports mirroring [raid1]