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

[Christoph Hellwig]

On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote:
> On Tue, May 12, 2026 at 07:20:41AM +0200, Christoph Hellwig wrote:
> > While the RAID6 algorithm could in theory support 3 devices by just
> > copying the data disk to the two parity disks, this version is not only
> > useless because it is a suboptimal version of 3-way mirroring, but also
> > broken with various crashes and incorrect parity generation in various
> > architecture-optimized implementations.  Disallow it similar to mdraid
> > which requires at least 4 devices for RAID 6.
> > 
> > Fixes: 53b381b3abeb ("Btrfs: RAID5 and RAID6")
> > Signed-off-by: Christoph Hellwig <hch at lst.de>
> 
> This patch should have been sent separately as it has user visible
> impact and can potentially break some setups.

It _is_ sent out separate.

> 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.