######################################## df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdd1        11T  2.8T  4.6T  38% /mnt/a
######################################## btrfs filesystem show
Label: none  uuid: 939881d5-cff0-4395-8fdd-3c23d64da5a5
        Total devices 3 FS bytes used 2.71TiB
        devid    1 size 7.28TiB used 2.71TiB path /dev/sdd1
        devid    2 size 7.28TiB used 22.01GiB path /dev/loop0
        devid    3 size 7.28TiB used 2.69TiB path /dev/sdf1

######################################## btrfs filesystem df
Data, RAID1: total=2.66TiB, used=2.66TiB
System, RAID1: total=8.00MiB, used=416.00KiB
Metadata, RAID1: total=57.00GiB, used=54.21GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
######################################## btrfs filesystem usage
Overall:
    Device size:                  21.83TiB
    Device allocated:              5.43TiB
    Device unallocated:           16.40TiB
    Device missing:                  0.00B
    Used:                          5.42TiB
    Free (estimated):              8.20TiB      (min: 8.20TiB)
    Data ratio:                       2.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,RAID1: Size:2.66TiB, Used:2.66TiB
   /dev/sdd1       2.66TiB
   /dev/sdf1       2.66TiB

Metadata,RAID1: Size:57.00GiB, Used:54.21GiB
   /dev/loop0     22.00GiB
   /dev/sdd1      55.00GiB
   /dev/sdf1      37.00GiB

System,RAID1: Size:8.00MiB, Used:416.00KiB
   /dev/loop0      8.00MiB
   /dev/sdd1       8.00MiB

Unallocated:
   /dev/loop0      7.26TiB
   /dev/sdd1       4.57TiB
   /dev/sdf1       4.58TiB
######################################## btrfs device usage
/dev/loop0, ID: 2
   Device size:             7.28TiB
   Device slack:              0.00B
   Metadata,RAID1:         22.00GiB
   System,RAID1:            8.00MiB
   Unallocated:             7.26TiB

/dev/sdd1, ID: 1
   Device size:             7.28TiB
   Device slack:              0.00B
   Data,RAID1:              2.66TiB
   Metadata,RAID1:         55.00GiB
   System,RAID1:            8.00MiB
   Unallocated:             4.57TiB

/dev/sdf1, ID: 3
   Device size:             7.28TiB
   Device slack:              0.00B
   Data,RAID1:              2.66TiB
   Metadata,RAID1:         37.00GiB
   Unallocated:             4.58TiB