/home/andrew/
[root@neptune:andrew]# btrfs device usage /mnt/sdb2
/dev/sdb2, ID: 1
   Device size:           213.00GiB
   Device slack:              0.00B
   Data,RAID1:            209.97GiB
   Metadata,RAID1:          3.00GiB
   System,RAID1:           32.00MiB
   Unallocated:             1.00MiB

missing, ID: 2
   Device size:               0.00B
   Device slack:              0.00B
   Data,RAID1:            209.97GiB
   Metadata,RAID1:          3.00GiB
   System,RAID1:           32.00MiB
   Unallocated:             1.00MiB

/home/andrew/
[root@neptune:andrew]#
/home/andrew/
[root@neptune:andrew]# btrfs filesystem usage /mnt/sdb2
Overall:
    Device size:                 426.00GiB
    Device allocated:            426.00GiB
    Device unallocated:            2.00MiB
    Device missing:              213.00GiB
    Used:                        403.83GiB
    Free (estimated):              9.34GiB      (min: 9.34GiB)
    Data ratio:                       2.00
    Metadata ratio:                   2.00
    Global reserve:              344.02MiB      (used: 0.00B)

Data,RAID1: Size:209.97GiB, Used:200.62GiB
   /dev/sdb2     209.97GiB
   missing       209.97GiB

Metadata,RAID1: Size:3.00GiB, Used:1.29GiB
   /dev/sdb2       3.00GiB
   missing         3.00GiB

System,RAID1: Size:32.00MiB, Used:48.00KiB
   /dev/sdb2      32.00MiB
   missing        32.00MiB

Unallocated:
   /dev/sdb2       1.00MiB
   missing         1.00MiB
/home/andrew/
[root@neptune:andrew]# btrfs filesystem show /mnt/sdb2
Label: none  uuid: 230b203c-fba5-4297-af02-c0a81bc6a2cf
        Total devices 2 FS bytes used 201.91GiB
        devid    1 size 213.00GiB used 213.00GiB path /dev/sdb2
        *** Some devices missing

/home/andrew/
[root@neptune:andrew]# btrfs filesystem df /mnt/sdb2
Data, RAID1: total=209.97GiB, used=200.62GiB
System, RAID1: total=32.00MiB, used=48.00KiB
Metadata, RAID1: total=3.00GiB, used=1.29GiB
GlobalReserve, single: total=344.02MiB, used=0.00B