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

[H. Peter Anvin]

On May 14, 2026 9:37:05 PM PDT, Christoph Hellwig <hch@xxxxxx> 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 don't think this is a good idea. Error out; it is the btrfs maintainers' job to ensure user data isn't lost. 

The RAID-6 code has *never* supported only 3 units, and if it ever worked for *any* of the implementations it was purely by accident. Speaking as the original author I should know; this was deliberate as in some cases the degenerate case (3) would have required extra trays in the code to no user benefit. 

I would not be surprised if the kernel crashed or corrupted the page cache in that case.