Btrfs says disk is full when there should be plenty of free space

Question

# df -H
Filesystem                            Size  Used Avail Use% Mounted on
...
/dev/bcache4                          8.1T  1.9T     0 100% /mnt/8t4

# btrfs fi df -H /mnt/8t4
Data, single: total=1.84TB, used=1.82TB
System, DUP: total=33.55MB, used=212.99kB
Metadata, DUP: total=2.15GB, used=1.91GB
GlobalReserve, single: total=536.87MB, used=0.00B

# btrfs fi usage /mnt/8t4
Overall:
    Device size:                   7.28TiB
    Device allocated:              1.67TiB
    Device unallocated:            5.60TiB
    Device missing:                  0.00B
    Used:                          1.66TiB
    Free (estimated):              5.62TiB      (min: 2.81TiB)
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,single: Size:1.67TiB, Used:1.66TiB (99.26%)
   /dev/bcache4    1.67TiB

Metadata,DUP: Size:2.00GiB, Used:1.78GiB (88.98%)
   /dev/bcache4    4.00GiB

System,DUP: Size:32.00MiB, Used:208.00KiB (0.63%)
   /dev/bcache4   64.00MiB

Unallocated:
   /dev/bcache4    5.60TiB

I've tried various balances but they didn't help. There are two fairly recent snapshots on the fs.

How do I make Btrfs realize there's ~6TB of free space?

Answer 1

"General linux userspace tools such as df will inaccurately report free space on a Btrfs partition. It is recommended to use btrfs filesystem usage to query Btrfs partitions." https://wiki.archlinux.org/index.php/Btrfs

Output of btrfs fi usage shows 5.6TB as available, what looks resonable.

Explanations about free space - https://btrfs.wiki.kernel.org/index.php/FAQ#How_much_free_space_do_I_have.3F

Answer 2

The problem was solved by rebooting into an updated kernel. Going from 5.4.8 to 5.6.7.