Re: [PATCH 08/18] raid6: warn when using less than four devices

[H. Peter Anvin]

On 2026-05-17 22:17, Christoph Hellwig wrote:
> Quoting H. Peter Anvin who came up with the RAID6 P/Q algorithm, and
> who wrote the initial implementation, then still part of the md driver:
> 
>   The RAID-6 code has *never* supported only 3 units, and if it ever
>   worked for *any* of the implementations it was purely by accident.
>   Speaking as the original author I should know; this was deliberate as
>   in some cases the degenerate case (3) would have required extra trays

Stupid autocorrect. That was of course supposed to be "tests" (as in extra
code paths) not "trays" :)

>   in the code to no user benefit.
> 
> While md never allowed less than 4 devices, btrfs does.  This new
> warning will trigger for such file systems, but given how it already
> causes havoc that is a good thing.  If btrfs wants to fix third, it
> should switch to transparently use three-way mirroring underneath,
> which will work as P and Q are copies of the single data device by
> the definition of the Linux RAID 6 P/Q algorithm.

For what it's worth, this is also true in the degenerate two-drive RAID-4|5
case (D = P).

	-hpa