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

    btrfs device add /dev/sdb1 /

  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

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

  5. Start balancing data and metadata in raid1 mode

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

  6. 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

Another way

Replaces steps 3 - 5

  1. Before mounting 

    mkfs.btrfs --data raid1 --metadata raid1 /dev/sda1 /dev/nvme0n1p3

  2. Mount to folder

    mount /dev/sda1 /data