On Wed, May 13, 2026 at 07:47:42AM +0200, Christoph Hellwig wrote:
On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote:
quoted
On Tue, May 12, 2026 at 07:20:41AM +0200, Christoph Hellwig wrote:
quoted
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@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.
It's public interface change of btrfs but in a patch series cleaning
up some library code, I noticed it by accident.
quoted
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.
The calcuation is a different than what I'm concened about, changing
minimum devices from 3 to 4 is a breaking change. If the library won't
provide the xor/parity functions then we'll have to add a fallback for
this special case.