From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 75793CD37B6 for ; Wed, 13 May 2026 05:59:33 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [127.0.0.1]) by lists.ozlabs.org (Postfix) with ESMTP id 4gFjTc0F84z2xpp; Wed, 13 May 2026 15:59:32 +1000 (AEST) Authentication-Results: lists.ozlabs.org; arc=none smtp.remote-ip=213.95.11.211 ARC-Seal: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1778651971; cv=none; b=BN607WNWKmvenqH7z+3cDXBq+9KHcPT1Kob/7q1IJlyaHewSk6+a4Qbph1OsqwFlsrr0Zld77sLkqnPrxBGlgF5QydRrcOnaax0RW0P98CLmYcnlOwRiNrcDjjl+dQNRn7HBl8lsBiXZhVytkeVzXrjHy/bzMP/jY76Rt9J3bLsj94tblJ+e+4G4/VpLKsZkjThzULHFrcniuYqF5UL7HI/4SAwpJTQDhrB02qL9DeLgjthtkN2FAAxA71H42EagcNduE1EQBzUWhZ+nMMmDc0CuI+nGaRmp/cJAaa514iENswv841+uE0W5GwCKOjm8HfuCmdoQ1eLJWk8Q4q5kXg== ARC-Message-Signature: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1778651971; c=relaxed/relaxed; bh=+doqOq5qahrITQTt42Vk0H3M5xERwzM2ccixxL3hjQo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ZOS6JVVz8zOQTSnyqRhwm6LFSRol7ydZexc+Y+7fRQNqJb98pGf5+Dg7u+umDOXC5e7oK6auFnerbXyVZnPUcYrPnvIE3QvwANhaWIjZwklpXkCh+eVxY3ufpff9+KaswBy/RaaNXFu+EZPcjJnYkfn8HgN/Kn5hBnvaGk3kEh/SfeYFxxFImNYQh6IocPAFjYDK2WR/WZoK1yf6CwVjKUGO2d9qbkSl3FGEyqNGrYbeJxP+bePzPBk8RoBWVrEDBd8M9qnvIHrHa3eaDPbYh85JcOPhYfbF9qVx47/XVuI6GaVvaUMAxb8nqCmubQw26HIhwSor6ksiJeE4bLDh5g== ARC-Authentication-Results: i=1; lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=lst.de; spf=pass (client-ip=213.95.11.211; helo=verein.lst.de; envelope-from=hch@lst.de; receiver=lists.ozlabs.org) smtp.mailfrom=lst.de Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=lst.de Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=lst.de (client-ip=213.95.11.211; helo=verein.lst.de; envelope-from=hch@lst.de; receiver=lists.ozlabs.org) X-Greylist: delayed 699 seconds by postgrey-1.37 at boromir; Wed, 13 May 2026 15:59:29 AEST 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 lists.ozlabs.org (Postfix) with ESMTPS id 4gFjTY5Mgxz2xlV for ; Wed, 13 May 2026 15:59:29 +1000 (AEST) 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> 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: <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.