From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.zytor.com (terminus.zytor.com [198.137.202.136]) (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 D576837B3F2; Fri, 15 May 2026 20:44:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778877877; cv=none; b=GIse8AIZxvQ2dIvzBBOlXUhU1i9BP8SRDT7dYWRHv4LQNwaA2wjdygOzcRHe8xs16XFKaXfZNjgNu0pu5GYTo+bijWpQA2fMqBP1UkUceMOIQx6QCDTGcCNKIT/8eudJOJLxsyM8vEdFXi3tE2dIarGteO9Bo5iOebbNSEr36lM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778877877; c=relaxed/simple; bh=E1+eWqDxv6j99dZMDhlO+NNDH9oYnsJmpH6o+qF/hR0=; h=Date:From:To:CC:Subject:In-Reply-To:References:Message-ID: MIME-Version:Content-Type; b=eDo4fU69+2vwDwBTZ97muihXaLNI6OKn1n/sVAzdNxrrD6+3fsQcgFkaLbV3wMLPNjrTxRvy0QQG5gCJDjV72qY35hBbXU65eCsmMMyFnXRr3IQJIlv1aa92e17KhGWEi7wyG3XZ8GnXnGMCgSM4/ivpSoEc9POhwS/oyXc90Ro= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com; spf=pass smtp.mailfrom=zytor.com; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b=HvwBqnLT; arc=none smtp.client-ip=198.137.202.136 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zytor.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=zytor.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=zytor.com header.i=@zytor.com header.b="HvwBqnLT" Received: from ehlo.thunderbird.net (c-76-133-66-138.hsd1.ca.comcast.net [76.133.66.138]) (authenticated bits=0) by mail.zytor.com (8.18.1/8.17.1) with ESMTPSA id 64FJxdt93762934 (version=TLSv1.3 cipher=TLS_AES_128_GCM_SHA256 bits=128 verify=NO); Fri, 15 May 2026 12:59:40 -0700 DKIM-Filter: OpenDKIM Filter v2.11.0 mail.zytor.com 64FJxdt93762934 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zytor.com; s=2026042601; t=1778875185; bh=yOPbK2OFF6IcrfoJraZaWua7RTa6T7n9+bq8VLS6qCY=; h=Date:From:To:CC:Subject:In-Reply-To:References:From; b=HvwBqnLT38+P8wOr09I0oo/MRbA0gltt1BvxpDB/CXoHAI/p9iMtZP4Acni9CBE8i UuQANyZJcLZO9XcaPLGm0i1Xg+soCjS4z5NjIoUETnDDa930DiHBVRAOT/MRlLb9jY QYrQ9QMS7lPYMHGvM8plKfNxe30Gl9az6f+vOCv0hNim3O4/Bdo2qSTnUxGO9V7Nx4 F46aEk0Mx5DZE0TAlF3l22P0Yw61+alulS+V4O2YtDdYTlzt2T6qIuwD2KE0nXfR9T jNDuDCJ8yXY/CId0JmSkw1Ar6+GJQTs5O0RqwrWv5l7cMbOBEgdh93GvzIYQG1gbNB I9HJmsdG2PKng== Date: Fri, 15 May 2026 12:59:34 -0700 From: "H. Peter Anvin" To: Christoph Hellwig , kreijack@inwind.it CC: David Sterba , 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, 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 User-Agent: K-9 Mail for Android In-Reply-To: <20260515043705.GA3855@lst.de> References: <20260512052230.2947683-1-hch@lst.de> <20260512052230.2947683-2-hch@lst.de> <20260512114231.GG2558453@suse.cz> <20260513054742.GA1018@lst.de> <0a8d1ff4-f5a2-49e9-aa45-d25dbe4ded40@libero.it> <20260515043705.GA3855@lst.de> Message-ID: <34C16854-1065-4542-8836-DDED58EC1844@zytor.com> Precedence: bulk X-Mailing-List: linux-raid@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On May 14, 2026 9:37:05 PM PDT, Christoph Hellwig wrote: >On Thu, May 14, 2026 at 09:51:59PM +0200, Goffredo Baroncelli wrote: >> I think that the David concern is : "what happens for an already >> existing btrfs raid6 3 disks filesystem when the user upgrade the kerne= l ?" >> (I am thinking when a new BG needs to be allocated)=2E=2E=2E > >Then it will cleanly fail to mount instead of constantly corrupting data >and memory with every write, yes=2E Which clearly suggest that such >file systems don't exist in the wild=2E > >But if btrfs wants to keep supporting this I'll just add a _unsafe >version without the check in the core library=2E I don't think this is a good idea=2E Error out; it is the btrfs maintainer= s' job to ensure user data isn't lost=2E=20 The RAID-6 code has *never* supported only 3 units, and if it ever worked = for *any* of the implementations it was purely by accident=2E Speaking as t= he original author I should know; this was deliberate as in some cases the = degenerate case (3) would have required extra trays in the code to no user = benefit=2E=20 I would not be surprised if the kernel crashed or corrupted the page cache= in that case=2E