Why do I need 4 disks for a raid6?

[Goswin von Brederlow <goswin-v-b@web.de>]

Hi,

I'm wondering why the kernel requires a raid6 to have at least 4
disks (of which at most 2 can be missing). Why not 3 disks?

I know a 1+2 disk raid6 sounds stupid but it has an use case. Let me
demonstrate this with raid5:

Lets say you started with a raid1 when you bought your computer:
mdadm --create -l 1 -n 2 /dev/md9 /dev/sda1 /dev/sdb1
a fewyears of use on /dev/md9 

Now you run out of space and went and bought a 3rd harddisk. What to
do now?
mdadm --stop /dev/md9
mdadm --create -l 5 -n 2 --assume-clean /dev/sda1 /dev/sdb1
mdadm --add /dev/md9 /dev/sdc1
mdadm --grow -n 3 /dev/md9

Voila. A nice raid5 with twice the size of the raid1. And no single
point of failure like degrading the raid1 and copying the data to a
degraded raid5 would have. And the downtime is just a minute to create
the new superblock.



Now for the raid6 case. With only 1 data disk and 2 parity disks all 3
disks should end up with identical data on them. In effect this should
be a 3 disk raid1, a cpu intensive one. Take an existing raid1 with 2
or 3 disks, stop the raid, create a new raid6 ovver it with
--assume-clean, start the raid. After that one can add more disks and
--grow -n 4/5/6/.. the raid6 to a sensible size. Again without going
into degraded mode.


So back to my original question: Why does the kernel require 4 disks
for a raid6 instead of allowing 3?

MfG
        Goswin

PS: Please CC me on replies.