linux-kernel - btrfs: Kernel warning when using/mount RAID 5/6

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Date: Sun, 17 Mar 2024 18:38:55 +0100
From: Diederik de Haas <didi.debian@...ow.org>
To: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
 David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Adam Borowski <kilobyte@...band.pl>,
 Nicholas D Steeves <sten@...ian.org>,
 Debian kernel ML <debian-kernel@...ts.debian.org>
Subject: btrfs: Kernel warning when using/mount RAID 5/6

Hi,

Since https://bugs.debian.org/863290 (2017) the Debian kernel has had a
patch to warn about the use of RAID 5/6 with BTRFS.
That bug mentions "It looks like there's a consensus that such a warning
should live in the kernel rather than userland"

Via [1] and [2] it seems userland did get a warning. It is mentioned in
the kernel documentation (``Documentation/btrfs-man5.rst``) (and [3]
and [4]), but AFAICT users are still not warned by the kernel when
using RAID 5/6.

I stumbled upon this issue when I was (locally) rebasing the Debian kernel
patch for kernel 6.8. I attached my rebased version of the original patch.
(I may have done it completely wrong as I know very little about BTRFS.)

But more importantly, IMO such a warning shouldn't be in a downstream
kernel (Debian), but in the upstream kernel source?

Cheers,
  Diederik

[1] https://lore.kernel.org/linux-btrfs/20161208153004.GA31795@angband.pl/
[2] https://lore.kernel.org/linux-btrfs/bf9594ea55ce40af80548888070427ad97daf78a.1598374255.git.josef@toxicpanda.com/
[3] https://btrfs.readthedocs.io/en/latest/btrfs-man5.html#raid56-status-and-recommended-practices
[4] https://lore.kernel.org/linux-btrfs/dbf47c42-932c-9cf0-0e50-75f1d779d024@cryptearth.de/
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