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.ga31...@angband.pl/ [2] https://lore.kernel.org/linux-btrfs/bf9594ea55ce40af80548888070427ad97daf78a.1598374255.git.jo...@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-9cf0-0e50-75f1d779d...@cryptearth.de/
From: Adam Borowski <kilob...@angband.pl> Date: Tue, 28 Mar 2017 16:55:05 +0200 Subject: btrfs: warn about RAID5/6 being experimental at mount time Bug-Debian: https://bugs.debian.org/863290 Origin: https://bugs.debian.org/863290#5 Too many people come complaining about losing their data -- and indeed, there's no warning outside a wiki and the mailing list tribal knowledge. Message severity chosen for consistency with XFS -- "alert" makes dmesg produce nice red background which should get the point across. Signed-off-by: Adam Borowski <kilob...@angband.pl> [bwh: Also add_taint() so this is flagged in bug reports] [2023-01-10: still accurate according to btrfs-progs own manpage: https://git.kernel.org/pub/scm/linux/kernel/git/kdave/btrfs-progs.git/commit/?id=922797e15590b836e377d6dc47b828356cafc2a9] [2024-03-17: still accurate; manpage is now in Documentation/btrfs-man5.rst implementation went from disk-io.c to super.c] --- fs/btrfs/super.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index 101f786963d4..2c409bce1bf5 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -731,6 +731,18 @@ static void set_device_specific_options(struct btrfs_fs_info *fs_info) !fs_info->fs_devices->rotating) btrfs_set_opt(fs_info->mount_opt, SSD); + /* + * Warn about RAID5/6 being experimental at mount time + */ + if ((fs_info->avail_data_alloc_bits | + fs_info->avail_metadata_alloc_bits | + fs_info->avail_system_alloc_bits) & + BTRFS_BLOCK_GROUP_RAID56_MASK) { + btrfs_alert(fs_info, + "btrfs RAID5/6 is EXPERIMENTAL and has known data-loss bugs"); + add_taint(TAINT_AUX, LOCKDEP_STILL_OK); + } + /* * For devices supporting discard turn on discard=async automatically, * unless it's already set or disabled. This could be turned off by
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