Add new raid1 btrfs filesystem

Assuming you have one btrfs partition used as root (/) and you want to create RAID 1 using another partition

  1. Check btrfs filesystem

    btrfs filesystem show

  2. Add another partition to root with force

    btrfs device add /dev/sdb1 / -f

  3. Check filesystem

    btrfs filesystem show

    Result is something like

    Label: none  uuid: 2601731d-934b-4c18-90fa-aa9cc61ced98
Total devices 2 FS bytes used 6.47GiB
devid    1 size 119.24GiB used 10.02GiB path /dev/sda2
devid    2 size 119.24GiB used 0.00B path /dev/sdb1

  4. Start RAID1 in root (/)

    btrfs balance start -dconvert=raid1 -mconvert=raid1 /

  5. Check file system usage

    btrfs filesystem df /

    When balancing is going on there is warning

    WARNING: Multiple block group profiles detected, see 'man btrfs(5)'
WARNING:    Data: single, raid1