how to check free/used storage of btrfs

cross-posted from: https://programming.dev/post/12030162

While checking for used and free space in a btrfs subvolume, I'm not getting a consistent value. It's confusing and doesn't help.

1. What is the correct way to find used/free space?
2. Why are these values inconsistent (except normal du)?

---

According to btrfs fi usage /home, 83.21 GiB is used.

Overall:
    Device size:		 149.98GiB
    Device allocated:		 100.07GiB
    Device unallocated:		  49.91GiB
    Device missing:		     0.00B
    Device slack:		     0.00B
    Used:			  83.21GiB
    Free (estimated):		  63.06GiB	(min: 38.10GiB)
    Free (statfs, df):		  63.06GiB
    Data ratio:			      1.00
    Metadata ratio:		      2.00
...

As per btrfs fi df /home, used space is 82.86 GiB, not 83.21 GiB.

Data, single: total=96.01GiB, used=82.86GiB
System, DUP: total=32.00MiB, used=16.00KiB
Metadata, DUP: total=2.00GiB, used=178.61MiB
GlobalReserve, single: total=99.50MiB, used=0.00B

As per btrfs fi du -s /home , used space is 63.11 GiB.

     Total   Exclusive  Set shared  Filename
  63.11GiB    13.64GiB    49.01GiB  /home

While according to du -hs /home, 64GiB is used.

---

Also, maximum space used should be close to 72 GiB as per btrfs fi du -s / and 73 GiB as per du -hs /, if btrfs fi usage includes all subvolumes . '/home' and '/' are on separate subvolumes.