On May 14, 2026 9:37:05 PM PDT, Christoph Hellwig [off-list ref] wrote:
On Thu, May 14, 2026 at 09:51:59PM +0200, Goffredo Baroncelli wrote:
quoted
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.