From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from verein.lst.de (verein.lst.de [213.95.11.211]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B628C3803D3 for ; Wed, 13 May 2026 05:47:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.95.11.211 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778651271; cv=none; b=tPftFmw24zxcsRvwIePzOv3XVC106kYQ8eV+bScS/vIeRRI9cfSZLGwIMdOA3snY0eAhs6YzdV3YyTn9/dG1J2ptnckbHwEWRZPtiEwS5c4am8spcMyF4b7gC/VjHaNjcBRkPcsVumaRXANdSuVMvxKR5+Bh/LXIo0+6LkuF3bA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778651271; c=relaxed/simple; bh=gLgo4WQ3hzDs13tL91Gp16GFz3W0lr9TsHhf12nzTgo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=G09e5YId/F0nMZWuSjaEdCCaPCN6fFR2ovs7PY8Pg6sSSwOqqFf7ZUP9p/Az5/7K78ON/VQiMvkCa+LVg8+ydO5NcmCVZpbcDDUuuDF63NhpY14I/euYia/WgU6t5gPqJYLeLjZ/v03anoaHAJJmuKOCMmYsHrs5vpdpfOqqo3k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=lst.de; spf=pass smtp.mailfrom=lst.de; arc=none smtp.client-ip=213.95.11.211 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=lst.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=lst.de Received: by verein.lst.de (Postfix, from userid 2407) id B404568B05; Wed, 13 May 2026 07:47:42 +0200 (CEST) Date: Wed, 13 May 2026 07:47:42 +0200 From: Christoph Hellwig To: David Sterba Cc: 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, "H. 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: <20260513054742.GA1018@lst.de> References: <20260512052230.2947683-1-hch@lst.de> <20260512052230.2947683-2-hch@lst.de> <20260512114231.GG2558453@suse.cz> Precedence: bulk X-Mailing-List: loongarch@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260512114231.GG2558453@suse.cz> User-Agent: Mutt/1.5.17 (2007-11-01) 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. > 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.