Re: [PATCH 01/19] btrfs: require at least 4 devices for RAID 6

From: Andrew Morton <akpm@linux-foundation.org>
To: Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc: Christoph Hellwig <hch@lst.de>, "H. Peter Anvin" <hpa@zytor.com>,
	kreijack@inwind.it, David Sterba <dsterba@suse.cz>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, Herbert Xu <herbert@gondor.apana.org.au>,
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	David Sterba <dsterba@suse.com>, Arnd Bergmann <arnd@arndb.de>,
	Song Liu <song@kernel.org>,
	Yu Kuai
	<yukuai@alb-78bjiv52429oh8qptp.cn-shenzhen.alb.aliyuncs.com>,
	Li Nan <linan122@huawei.com>,
	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
Date: Thu, 21 May 2026 17:17:30 -0700	[thread overview]
Message-ID: <20260521171730.7872482df453975cf60ce7dc@linux-foundation.org> (raw)
In-Reply-To: <f46636c8-80ba-4802-a6a0-74cbc35e7bee@gmx.com>

On Wed, 20 May 2026 18:11:09 +0930 Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:

> 
> 
> 在 2026/5/18 14:42, Christoph Hellwig 写道:
> > On Fri, May 15, 2026 at 12:59:34PM -0700, H. Peter Anvin wrote:
> >> I don't think this is a good idea. Error out; it is the btrfs maintainers' job to ensure user data isn't lost.
> >>
> >> 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. Speaking as the 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.
> >>
> >> I would not be surprised if the kernel crashed or corrupted the page cache in that case.
> > 
> > It does, that's why I wanted to exclude it.  Anyway, for the about to be
> > resent version I'll drop this btrfs patch over the stated objection and
> > will otherwise not change anything.  This means the (IMHO hypothetical)
> > users of this configuration will get a WARN_ON_ONCE triggered, but
> > otherwise keep working (or rather not working) as before.
> > 
> 
> For the btrfs part, I believe I can get the current 2-disk-raid5 and 
> 3-disk-raid6 to fallback to raid1 inside btrfs.
> 
> I hope the btrfs part can be finished and reach the next merge window, 
> but I'm not 100% sure.
> 
> What is the planned cycle to merge this raid5/6 cleanup?

At present it's on track for the 7.2-rc1 merge window.  Does that suit?