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

[Goffredo Baroncelli]

On 15/05/2026 06.37, Christoph Hellwig wrote:
> On Thu, May 14, 2026 at 09:51:59PM +0200, Goffredo Baroncelli wrote:
>> 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)...
> 
> Then it will cleanly fail to mount instead of constantly corrupting data
> and memory with every write, yes.  Which clearly suggest that such
> file systems don't exist in the wild.
> 
> But if btrfs wants to keep supporting this I'll just add a _unsafe
> version without the check in the core library.
> 

I am not arguing about this part. My point is that the change shouldn't have impacted the
BTRFS interface versus the user (as patch 01/19 does), but instead the change should
have modify the interface raid code <-> btrfs (e.g. doing a memcpy....), or at least the
cover letter should warn that the raid6 code requires a number of disk >= 4, pointing
to BTRFS as "client doing wrong things".

At least, the message was received: don't relay to the raid6 code when the number of disk is
less than 4.

BR
GB

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