# btrfs fi usage -t /mnt
Overall:
    Device size:                          18.19TiB
    Device allocated:                      1.59TiB
    Device unallocated:                   16.61TiB
    Device missing:                        0.00TiB
    Used:                                  1.58TiB
    Free (estimated):                     16.61TiB      (min: 8.31TiB)
    Data ratio:                               1.00
    Metadata ratio:                           2.00
    Global reserve:                        0.00TiB      (used: 0.00TiB)

Data,RAID0: Size:1.58TiB, Used:1.57TiB
   /dev/sdaa       0.79TiB
   /dev/sdab       0.79TiB

Metadata,RAID1: Size:0.00TiB, Used:0.00TiB
   /dev/sdaa       0.00TiB
   /dev/sdab       0.00TiB

System,RAID1: Size:0.00TiB, Used:0.00TiB
   /dev/sdaa       0.00TiB
   /dev/sdab       0.00TiB

Unallocated:
   /dev/sdaa       8.30TiB
   /dev/sdab       8.30TiB