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

- From: H. Peter Anvin \<hpa@zytor.com\>
- Date: 2026-05-14 20:11:10 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2026/05/9007728
- Thread: https://ratatoskr.run/linux-arm-kernel/2026/05/8997837/t.md

```text
On May 14, 2026 12:51:59 PM PDT, Goffredo Baroncelli [ref] wrote:
>On 13/05/2026 07.47, Christoph Hellwig wrote:
>> On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote:
>
>> 
>>> 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.
>> 
>
>I think that the David concern is : "what happens for an already
>existing btrfs raid6 3 disks filesystem when the user upgrade the kernel ?"
>(I am thinking when a new BG needs to be allocated)...
>
>BR
>GB
>

That's what I'm saying – it should invoke the RAID-1 code under the cover (as with 3 disks, D = P = Q.)

```