btrfs fi usage /
Overall:
    Device size:		 128.00GiB
    Device allocated:		  90.06GiB
    Device unallocated:		  37.93GiB
    Device missing:		     0.00B
    Used:			  86.32GiB
    Free (estimated):		  19.75GiB	(min: 19.75GiB)
    Data ratio:			      2.00
    Metadata ratio:		      2.00
    Global reserve:		 123.27MiB	(used: 0.00B)

Data,RAID1: Size:43.00GiB, Used:42.22GiB
   /dev/sdb1	  29.00GiB
   /dev/sdc1	  27.00GiB
   /dev/sdd1	  28.00GiB
   /dev/sdg1	   2.00GiB

Metadata,RAID1: Size:2.00GiB, Used:964.47MiB
   /dev/sdb1	   1.00GiB
   /dev/sdc1	   2.00GiB
   /dev/sdd1	   1.00GiB

System,RAID1: Size:32.00MiB, Used:16.00KiB
   /dev/sdb1	  32.00MiB
   /dev/sdc1	  32.00MiB

Unallocated:
   /dev/sdb1	   1.97GiB
   /dev/sdc1	   2.97GiB
   /dev/sdd1	   3.00GiB
   /dev/sdg1	  30.00GiB