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Peter Anvin" , 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: <20260513201928.GO2558453@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> 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=us-ascii Content-Disposition: inline In-Reply-To: <20260513054742.GA1018@lst.de> 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: ED6E36B0F3 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 Wed, May 13, 2026 at 07:47:42AM +0200, Christoph Hellwig wrote: > On Tue, May 12, 2026 at 01:42:31PM +0200, David Sterba wrote: > > On Tue, May 12, 2026 at 07:20:41AM +0200, Christoph Hellwig wrote: > > > While the RAID6 algorithm could in theory support 3 devices by just > > > copying the data disk to the two parity disks, this version is not only > > > useless because it is a suboptimal version of 3-way mirroring, but also > > > broken with various crashes and incorrect parity generation in various > > > architecture-optimized implementations. Disallow it similar to mdraid > > > which requires at least 4 devices for RAID 6. > > > > > > Fixes: 53b381b3abeb ("Btrfs: RAID5 and RAID6") > > > Signed-off-by: Christoph Hellwig > > > > This patch should have been sent separately as it has user visible > > impact and can potentially break some setups. > > It _is_ sent out separate. It's public interface change of btrfs but in a patch series cleaning up some library code, I noticed it by accident. > > 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. The calcuation is a different than what I'm concened about, changing minimum devices from 3 to 4 is a breaking change. If the library won't provide the xor/parity functions then we'll have to add a fallback for this special case.