btrfs-raid0

sudo mount /dev/sda2 /
# mounting sda2 is boot time related via fstab:
# UUID=01e804ec-cc93-44c9-8b92-8fcd41c95aff  /  btrfs  rw,relatime,space_cache  0 0


# now add sdb1 to /
sudo btrfs device add /dev/sdb1 /

sudo btrfs filesystem show
# Label: none  uuid: 01e804ec-cc93-44c9-8b92-8fcd41c95aff
#        Total devices 2 FS bytes used 8.42GiB
#        devid    1 size 931.02GiB used 10.06GiB path /dev/sda2
#        devid    2 size 931.51GiB used 0.00B path /dev/sdb1


sudo btrfs balance start -dconvert=raid1 -mconvert=raid1 /
# take some time

sudo btrfs filesystem show
# Label: none  uuid: 01e804ec-cc93-44c9-8b92-8fcd41c95aff
#        Total devices 2 FS bytes used 8.42GiB
#        devid    1 size 931.02GiB used 4.06GiB path /dev/sda2
#        devid    2 size 931.51GiB used 6.00GiB path /dev/sdb1

sudo btrfs filesystem df /
# Data, single: total=9.00GiB, used=8.30GiB
# System, DUP: total=32.00MiB, used=16.00KiB
# Metadata, DUP: total=512.00MiB, used=125.62MiB
# GlobalReserve, single: total=48.00MiB, used=0.00B


# To delete a device (which can take some "seconds"
btrfs device delete /dev/sdb1 /