btrfstune -m breaks btrfs scrub

btrfstune-m-breaks-btrfs-scrub.sh

#!/bin/bash
# dlitz 2022
# This tests 'btrfstune -m' (change metadata_uuid) followed by 'btrfs scrub'.
# Apparently, changing the metadata_uuid causes btrfs scrub to spew
# uncorrectable errors, even on an empty filesystem.
# Changing the metadata_uuid back to the old value makes 'btrfs scrub' happy
# again.

set -eu

verbose() {
  printf >&2 '#'
  printf >&2 ' %q' "$@"
  printf >&2 '\n'
  "$@"
}

shverbose() {
  printf >&2 '# %s\n' "$1"
  eval "$1"
}

#verbose lsb_release -a
#shverbose 'dpkg -s btrfs-progs | grep ^Version'
verbose uname -srvmo
verbose mkfs.btrfs --version

tmp_dir=$(mktemp -d)

cd "$tmp_dir"

image="./fsimage.bin"
mountpoint="./mnt"

# create filesystem image
verbose truncate -s 8G "$image"

# Set up loop device
shverbose 'loopdev=$(losetup --find --show "$image" | tee /dev/fd/2)'

verbose mkfs.btrfs --quiet "$loopdev"
verbose mkdir "$mountpoint"

verbose mount -t btrfs "$loopdev" "$mountpoint"
verbose btrfs scrub start -B "$mountpoint"
verbose umount "$mountpoint"

shverbose 'old_uuid=$(blkid -o value -s UUID "$loopdev" | tee /dev/fd/2)'
verbose btrfstune -m "$loopdev"
shverbose 'new_uuid=$(blkid -o value -s UUID "$loopdev" | tee /dev/fd/2)'

verbose mount -t btrfs "$loopdev" "$mountpoint"
verbose btrfs scrub start -B "$mountpoint" || true
umount "$mountpoint"

shverbose "btrfstune -M \"\$old_uuid\" $loopdev"
verbose mount -t btrfs "$loopdev" "$mountpoint"
verbose btrfs scrub start -B "$mountpoint" || true
umount "$mountpoint"

# cleanup
rmdir "$mountpoint"
losetup -d "$loopdev"
rm -f "$image"
cd /
rmdir "$tmp_dir"
echo "# "

output.txt

# uname -srvmo
Linux 5.17.0-1-amd64 #1 SMP PREEMPT Debian 5.17.3-1 (2022-04-18) x86_64 GNU/Linux
# mkfs.btrfs --version
mkfs.btrfs, part of btrfs-progs v5.17
# truncate -s 8G ./fsimage.bin
# loopdev=$(losetup --find --show "$image" | tee /dev/fd/2)
/dev/loop5
# mkfs.btrfs --quiet /dev/loop5
NOTE: several default settings have changed in version 5.15, please make sure
      this does not affect your deployments:
      - DUP for metadata (-m dup)
      - enabled no-holes (-O no-holes)
      - enabled free-space-tree (-R free-space-tree)

# mkdir ./mnt
# mount -t btrfs /dev/loop5 ./mnt
# btrfs scrub start -B ./mnt
scrub done for 3ba6ccff-a888-400a-9d89-19eed3bea7ae
Scrub started:    Tue May 10 18:50:18 2022
Status:           finished
Duration:         0:00:00
Total to scrub:   536.00MiB
Rate:             0.00B/s
Error summary:    no errors found
# umount ./mnt
# old_uuid=$(blkid -o value -s UUID "$loopdev" | tee /dev/fd/2)
3ba6ccff-a888-400a-9d89-19eed3bea7ae
# btrfstune -m /dev/loop5
# new_uuid=$(blkid -o value -s UUID "$loopdev" | tee /dev/fd/2)
2e8822a7-560e-46f1-ba57-a7ff52dd5162
# mount -t btrfs /dev/loop5 ./mnt
# btrfs scrub start -B ./mnt
ERROR: there are uncorrectable errors
scrub done for 2e8822a7-560e-46f1-ba57-a7ff52dd5162
Scrub started:    Tue May 10 18:50:18 2022
Status:           finished
Duration:         0:00:00
Total to scrub:   536.00MiB
Rate:             0.00B/s
Error summary:    verify=18
  Corrected:      0
  Uncorrectable:  18
  Unverified:     0
# btrfstune -M "$old_uuid" /dev/loop5
# mount -t btrfs /dev/loop5 ./mnt
# btrfs scrub start -B ./mnt
scrub done for 3ba6ccff-a888-400a-9d89-19eed3bea7ae
Scrub started:    Tue May 10 18:50:19 2022
Status:           finished
Duration:         0:00:00
Total to scrub:   536.00MiB
Rate:             0.00B/s
Error summary:    no errors found
#