It's output looks like:
Label: 'test' uuid: d23f3138-c8ea-449e-9155-015ce4f6e2e6
Total devices 1 FS bytes used 131072
devid 1 size 981467136 used 252706816 path /dev/sdb1
FS bytes used number is actual data on btrfs volume. However,
to resize successfully we need to know actual space used by data
and metadata. That's why we read size from devid (i.e. 252706816).
Btrfs volumes can span over multiple devices, so we must make sure
that we are reading devid corresponding to the correct device node.
BUG: 353333
Andrius Štikonas
Teo Mrnjavac