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Peter Anvin" To: kreijack@inwind.it, Goffredo Baroncelli , Christoph Hellwig , David Sterba CC: 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 User-Agent: K-9 Mail for Android In-Reply-To: <0a8d1ff4-f5a2-49e9-aa45-d25dbe4ded40@libero.it> 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> Message-ID: <0507CCEF-0548-442F-8703-1D006B5E068B@zytor.com> Precedence: bulk X-Mailing-List: linux-raid@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On May 14, 2026 12:51:59 PM PDT, Goffredo Baroncelli = wrote: >On 13/05/2026 07=2E47, Christoph Hellwig wrote: >> On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote: > >>=20 >>> The degenerate modes of >>> raid0, 5, or 6 are explicit as a possible middle step when converting >>> profiles=2E We can use a fallback implementation for this case if the >>> accelerated implementations cannot do it=2E >>=20 >> This is not about a degenerated mode=2E For a degenerated RAID 6, pari= ty >> generation uses the RAID 5 XOR routines as the second parity will be >> missing=2E This is about generating two parities for a single data dis= k, >> which must be explicitly selected=2E >>=20 > >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)=2E=2E=2E > >BR >GB > That's what I'm saying =E2=80=93 it should invoke the RAID-1 code under th= e cover (as with 3 disks, D =3D P =3D Q=2E)