[PATCH 01/19] btrfs: require at least 4 devices for RAID 6

[Goffredo Baroncelli]

On 13/05/2026 07.47, Christoph Hellwig wrote:
> On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote:

> 
>> The degenerate modes of
>> raid0, 5, or 6 are explicit as a possible middle step when converting
>> profiles.  We can use a fallback implementation for this case if the
>> accelerated implementations cannot do it.
> 
> This is not about a degenerated mode.  For a degenerated RAID 6, parity
> generation uses the RAID 5 XOR routines as the second parity will be
> missing.  This is about generating two parities for a single data disk,
> which must be explicitly selected.
> 

I think that the David concern is : "what happens for an already
existing btrfs raid6 3 disks filesystem when the user upgrade the kernel ?"
(I am thinking when a new BG needs to be allocated)...

BR
GB

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