From: "H. Peter Anvin" <hpa@zytor.com>
To: Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>
Cc: 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>,
Dan Williams <dan.j.williams@intel.com>, Chris Mason <clm@fb.com>,
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 08/18] raid6: warn when using less than four devices
Date: Fri, 26 Jun 2026 18:14:11 -0700 [thread overview]
Message-ID: <33ef4e7b-3139-4cb8-9545-6d445bf6e5f1@zytor.com> (raw)
In-Reply-To: <20260518051804.462141-9-hch@lst.de>
On 2026-05-17 22:17, Christoph Hellwig wrote:
> Quoting H. Peter Anvin who came up with the RAID6 P/Q algorithm, and
> who wrote the initial implementation, then still part of the md driver:
>
> 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
Stupid autocorrect. That was of course supposed to be "tests" (as in extra
code paths) not "trays" :)
> in the code to no user benefit.
>
> While md never allowed less than 4 devices, btrfs does. This new
> warning will trigger for such file systems, but given how it already
> causes havoc that is a good thing. If btrfs wants to fix third, it
> should switch to transparently use three-way mirroring underneath,
> which will work as P and Q are copies of the single data device by
> the definition of the Linux RAID 6 P/Q algorithm.
For what it's worth, this is also true in the degenerate two-drive RAID-4|5
case (D = P).
-hpa
next prev parent reply other threads:[~2026-06-27 1:42 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-18 5:17 cleanup the RAID6 P/Q library v3 Christoph Hellwig
2026-05-18 5:17 ` [PATCH 01/18] raid6: turn the userspace test harness into a kunit test Christoph Hellwig
2026-06-26 8:20 ` patchwork-bot+linux-riscv
2026-05-18 5:17 ` [PATCH 02/18] raid6: remove __KERNEL__ ifdefs Christoph Hellwig
2026-05-18 5:17 ` [PATCH 03/18] raid6: move to lib/raid/ Christoph Hellwig
2026-05-18 5:17 ` [PATCH 04/18] raid6: remove unused defines in pq.h Christoph Hellwig
2026-05-18 5:17 ` [PATCH 05/18] raid6: remove raid6_get_zero_page Christoph Hellwig
2026-05-18 5:17 ` [PATCH 06/18] raid6: use named initializers for struct raid6_calls Christoph Hellwig
2026-05-18 5:17 ` [PATCH 07/18] raid6: improve the public interface Christoph Hellwig
2026-05-18 5:17 ` [PATCH 08/18] raid6: warn when using less than four devices Christoph Hellwig
2026-06-27 1:14 ` H. Peter Anvin [this message]
2026-05-18 5:17 ` [PATCH 09/18] raid6: hide internals Christoph Hellwig
2026-05-18 5:17 ` [PATCH 10/18] raid6: rework registration of optimized algorithms Christoph Hellwig
2026-05-18 5:17 ` [PATCH 11/18] raid6: use static_call for gen_syndrom and xor_syndrom Christoph Hellwig
2026-05-18 5:17 ` [PATCH 12/18] raid6: use static_call for raid6_recov_2data and raid6_recov_datap Christoph Hellwig
2026-05-18 5:17 ` [PATCH 13/18] raid6: update top of file comments Christoph Hellwig
2026-05-18 5:17 ` [PATCH 14/18] raid6_kunit: use KUNIT_CASE_PARAM Christoph Hellwig
2026-05-18 5:17 ` [PATCH 15/18] raid6_kunit: dynamically allocate data buffers using vmalloc Christoph Hellwig
2026-05-18 5:17 ` [PATCH 16/18] raid6_kunit: cleanup dataptr handling Christoph Hellwig
2026-05-18 5:18 ` [PATCH 17/18] raid6_kunit: randomize parameters and increase limits Christoph Hellwig
2026-05-18 5:18 ` [PATCH 18/18] raid6_kunit: randomize buffer alignment Christoph Hellwig
2026-05-18 21:12 ` cleanup the RAID6 P/Q library v3 Andrew Morton
2026-05-19 8:24 ` Christoph Hellwig
2026-06-27 0:52 ` H. Peter Anvin
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