Re: [PATCH 3/3] btrfs: disguise single-data-RAID56 as RAID1/RAID1C3

[David Sterba <dsterba@suse.cz>]

On Mon, May 25, 2026 at 08:00:30AM +0200, Christoph Hellwig wrote:
> On Sat, May 23, 2026 at 04:23:44PM +0200, David Sterba wrote:
> > On Fri, May 22, 2026 at 06:53:53PM +0930, Qu Wenruo wrote:
> > > Recently kernel RAID56 lib is trying to remove the unexpected
> > > single-data-RAID56 (2 disks RAID5 or 3 disk RAID5) support, meanwhile
> > > btrfs still supports such setup, which means in the long run btrfs has
> > > to handle such corner case by ourselves.
> > > 
> > > Thankfully single-data-RAID56 is really RAID1/RAID1C3, since data and
> > > P/Q stripes all match each other, rotation also makes no difference.
> > > 
> > > This patch will disguise those single-data-RAID56 chunks as
> > > RAID1/RAID1C3 chunks.
> > 
> > I don't think this is the right way to fix it. Calculations are an
> > implementation detail and should be done at the time of the xor_gen or
> > raid6_call, not touching the upper level structures related to format.
> 
> It absolutely is.  Adding fast path workarounds for this is completely
> stupid when it can be trivially handled on the mount side.

It's a lazy hack at best and fix on absolutely wrong layer. The library
should provide the support for the edge case. But we disagree on that.