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

[H. Peter Anvin]

On May 11, 2026 10:20:41 PM PDT, Christoph Hellwig <hch at lst.de> 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>
>---
> fs/btrfs/volumes.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>index a88e68f90564..0b54b97bdad8 100644
>--- a/fs/btrfs/volumes.c
>+++ b/fs/btrfs/volumes.c
>@@ -159,7 +159,7 @@ const struct btrfs_raid_attr btrfs_raid_array[BTRFS_NR_RAID_TYPES] = {
> 		.sub_stripes	= 1,
> 		.dev_stripes	= 1,
> 		.devs_max	= 0,
>-		.devs_min	= 3,
>+		.devs_min	= 4,
> 		.tolerated_failures = 2,
> 		.devs_increment	= 1,
> 		.ncopies	= 1,

Yes, if anyone cares about < 4 disks for the RAID-6 case (or < 3 for the RAID-4/5 case), just use the RAID-1 code.