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Peter Anvin" Cc: kreijack@inwind.it, Goffredo Baroncelli , Christoph Hellwig , Andrew Morton , Catalin Marinas , Will Deacon , Ard Biesheuvel , Huacai Chen , WANG Xuerui , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, Herbert Xu , Dan Williams , Chris Mason , David Sterba , Arnd Bergmann , Song Liu , Yu Kuai , Li Nan , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-crypto@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-arch@vger.kernel.org, linux-raid@vger.kernel.org Subject: Re: [PATCH 01/19] btrfs: require at least 4 devices for RAID 6 Message-ID: <20260515145158.GP2558453@suse.cz> Reply-To: dsterba@suse.cz References: <20260512052230.2947683-1-hch@lst.de> <20260512052230.2947683-2-hch@lst.de> <20260512114231.GG2558453@suse.cz> <20260513054742.GA1018@lst.de> <0a8d1ff4-f5a2-49e9-aa45-d25dbe4ded40@libero.it> <0507CCEF-0548-442F-8703-1D006B5E068B@zytor.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0507CCEF-0548-442F-8703-1D006B5E068B@zytor.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Spamd-Result: default: False [-4.00 / 50.00]; REPLY(-4.00)[] X-Rspamd-Queue-Id: 4B04666D5C X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action On Thu, May 14, 2026 at 12:57:53PM -0700, H. Peter Anvin wrote: > On May 14, 2026 12:51:59 PM PDT, Goffredo Baroncelli wrote: > >On 13/05/2026 07.47, Christoph Hellwig wrote: > >> On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote: > > > >> > >>> The degenerate modes of > >>> raid0, 5, or 6 are explicit as a possible middle step when converting > >>> profiles. We can use a fallback implementation for this case if the > >>> accelerated implementations cannot do it. > >> > >> This is not about a degenerated mode. For a degenerated RAID 6, parity > >> generation uses the RAID 5 XOR routines as the second parity will be > >> missing. This is about generating two parities for a single data disk, > >> which must be explicitly selected. > >> > > > >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)... > > That's what I'm saying – it should invoke the RAID-1 code under the cover (as with 3 disks, D = P = Q.) Thanks, it was not clear to me what you meant. For the two edge cases the code should do simple memcpy for both calculations of parity and recovery.