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 DD741CD4F21 for ; Wed, 13 May 2026 20:19:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:Reply-To:List-Subscribe:List-Help: List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=bxv4l5TDHn4zIZOm6RB+235uiCtzx6Q939PciMECEFA=; b=ybcAWy6giQ1Wem IEo3guHN2guyEfzGMUcRrn9DLUIVA09cP+b+MW5FjM7aePZp6RoQ/YLp+/HwJnU/sF/YZmojOQWqt lWZQ+WYQB2f7KYdxE7Vsd6ao19z3Ji1DDL8wzgm3OLYj2lq66S1NQfSNlo8tg0ulbvPxTonUAElzK myixV2P8arb4j/hI1hEGNdK4fKYKM6fL1EjK57t18SOuaxs3+KXYdkEyw8d0z7Eimbk460vYGqXC+ wTpi2WlS2Kzev4JHM6Xwuqnc1ickQqIUzmCSzYLdc584PdRIcysyPQaX1s+x+JPgZnlwb7j93joGK gXb9VUYYZ0oIVOCUwOrg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wNG3M-00000003gMX-0QF5; Wed, 13 May 2026 20:19:36 +0000 Received: from smtp-out1.suse.de ([195.135.223.130]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wNG3J-00000003gLK-2AKF for linux-riscv@lists.infradead.org; Wed, 13 May 2026 20:19:34 +0000 Received: from imap1.dmz-prg2.suse.org (imap1.dmz-prg2.suse.org [IPv6:2a07:de40:b281:104:10:150:64:97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id ED6E36B0F3; Wed, 13 May 2026 20:19:29 +0000 (UTC) Authentication-Results: smtp-out1.suse.de; none Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id A5E0E593A9; Wed, 13 May 2026 20:19:29 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id G7fLJ9HcBGpwcwAAD6G6ig (envelope-from ); Wed, 13 May 2026 20:19:29 +0000 Date: Wed, 13 May 2026 22:19:28 +0200 From: David Sterba To: Christoph Hellwig 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, "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: <20260513201928.GO2558453@suse.cz> References: <20260512052230.2947683-1-hch@lst.de> <20260512052230.2947683-2-hch@lst.de> <20260512114231.GG2558453@suse.cz> <20260513054742.GA1018@lst.de> MIME-Version: 1.0 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 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260513_131933_699148_1F792FE2 X-CRM114-Status: GOOD ( 27.10 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: dsterba@suse.cz Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org 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. _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv