Michael Makuch wrote:

I realize this is the developers list and though I am a developer I'm 
not a developer
of linux raid, but I can find no other source of answers to these questions:

Don't worry; it's a user list too.

$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 etherd/e0.0[0] etherd/e0.2[9](S) etherd/e0.9[8] 
etherd/e0.8[7] etherd/e0.7[6] etherd/e0.6[5] etherd/e0.5[4] 
etherd/e0.4[3] etherd/e0.3[2] etherd/e0.1[1]
      3907091968 blocks level 5, 64k chunk, algorithm 2 [9/9] [UUUUUUUUU]
      [============>........]  resync = 64.5% (315458352/488386496) 
finish=2228.0min speed=1292K/sec
unused devices: <none>

and I have no idea where the raid6 came from.

As far as I understand, the "Personalities" line just shows what RAID
capabilities are compiled into the kernel (and loaded, if modules). For
example, even though I'm only using raid0, I have:

-----------------------------------------------------------------
$ cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md0 : active raid0 sdc[0] sdb[1]
      976772992 blocks 64k chunks

unused devices: <none>
-----------------------------------------------------------------

-Corey