On May 14, 2026 9:37:05 PM PDT, Christoph Hellwig <[email protected]> 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.
