From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.223.130]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 815E33A7F4A for ; Wed, 13 May 2026 20:19:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=195.135.223.130 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778703573; cv=none; b=eSXPFZ1T1JjQv5TWOi77Hs6yoHqyyZcmQvNJ5AH69IVDI62URMe9FDyt1tgFYj8XyrSjvC5r6ajFK4lGXz5+X+IF/ixDtb+ADUSeVIq4mkNV19rjtTzI8Ggi9G9xtx6wkSGlJwPW9mFSVL53Kak47Ws9TIpOhD6Yels1hzV+pc0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778703573; c=relaxed/simple; bh=EDn4odiH74H3ifCfhNGOCtR8TgiezO1EVoWQXBXXRIY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=upM2Mzka5UA5fl6w7LJvwa8o8d/mJq66d2W414gUmYQ1SICeeP4zL/f2yLbTzfpH5EKygr4RF0B4LeLBqBbEfHEXeJCN3EmcHiMSEG8gHv9ydPqbNJYHisJrwpITFv6ibaabvw52XTwqSU2BVQw+k9nITithWhbHcRr4zZJa+e8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz; spf=pass smtp.mailfrom=suse.cz; arc=none smtp.client-ip=195.135.223.130 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=suse.cz 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> 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> Precedence: bulk X-Mailing-List: linux-crypto@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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-Spam-Score: -4.00 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-Spam-Flag: NO X-Spam-Level: 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.