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

[Christoph Hellwig]

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,
-- 
2.53.0