Re: Why do I need 4 disks for a raid6?

[Andre Noll <maan@systemlinux.org>]

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On 20:12, Goswin von Brederlow wrote:

> 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?

Yes, this limitation looks a bit arbitrary. I can not see any reason
why raid6 requires at least four disks. Probably it even works without
any significant changes if some of the checks are being relaxed.

> 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.

In fact, it wouldn't be much more CPU intensive than raid5 because
the math to "calculate" the Q parity would obviously not involve any
GF multiplications at all.

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

Dunno. Maybe Dan, Neil or HPA can tell the reason for imposing this
limitation.

Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe

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