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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 78B41CD37B6 for ; Wed, 13 May 2026 05:48:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=+doqOq5qahrITQTt42Vk0H3M5xERwzM2ccixxL3hjQo=; b=rq8gmEtJDdp9XDhLlKKjbisHRf y33KVVrJnnS0oaTK2BmBFvc76uI69Bm0dw2pYXfXzPVuOGlLIZ+yZcmdJvbxIl4kHJb3nTXmNJPY1 GwVIuk3x83qpv9/AvUNf5mouy6iRySCMRTNooemXb1jwOCOrPXb4xTMbZHS0p1fDG1PcSfM/hCJoY BpmadS3R4pZQHczQlUHf+r14psEf+PExGei0wc9qzyvuhGSlbIcBzOB5cNJCqIrj5woJYcQSLdcU3 jX0iBTxhiJI93+uBnFw9REZscqZdZfWkgZLLLG4r2JLH3tP4lWBmtkZVUX/p4TVd0wgDAyC2e9x28 MWrbyEDA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wN2Rm-00000001MhT-26fX; Wed, 13 May 2026 05:47:54 +0000 Received: from verein.lst.de ([213.95.11.211]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wN2Ri-00000001MgA-0qbk; Wed, 13 May 2026 05:47:52 +0000 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> 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) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260512_224750_380222_6E63D641 X-CRM114-Status: GOOD ( 22.48 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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.