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

[David Sterba]

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. 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.