<code>

# mkfs.btrfs -L data /dev/sda4
# btrfs filesystem show
# mount /dev/sda4 /data
# btrfs filesystem df /data
# btrfs filesystem usage /data
</code>


add new device
<code>
# btrfs device add /dev/xvdb /
# btrfs filesystem balance /
Done, had to relocate 7 out of 7 chunks

</code>