BTRFS with kernel 5.4.1: df -h shows 0 Bytes available but btrfs tells me that there are 1.06 TiB free

[Toralf FÃrster]

Since 5.4 (5.3.xy was fine) I do get every few days:

# df -h /home/tinderbox/img1
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda5       1.7T  614G     0 100% /home/tinderbox/img1


# btrfs filesystem usage /home/tinderbox/img1
Overall:
    Device size:                   1.66TiB
    Device allocated:            614.02GiB
    Device unallocated:            1.06TiB
    Device missing:                  0.00B
    Used:                        613.38GiB
    Free (estimated):              1.06TiB      (min: 543.50GiB)
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,RAID0: Size:566.00GiB, Used:565.50GiB (99.91%)
   /dev/sda5     283.00GiB
   /dev/sdb1     283.00GiB

Metadata,RAID1: Size:24.00GiB, Used:23.94GiB (99.76%)
   /dev/sda5      24.00GiB
   /dev/sdb1      24.00GiB

System,RAID1: Size:8.00MiB, Used:64.00KiB (0.78%)
   /dev/sda5       8.00MiB
   /dev/sdb1       8.00MiB

Unallocated:
   /dev/sda5     542.99GiB
   /dev/sdb1     542.99GiB


This is a hardened Gentoo linux acting as a QA build bot for their packages:

# uname -a
Linux mr-fox 5.4.2 #2 SMP Thu Dec 5 01:18:19 CET 2019 x86_64 Intel(R) Xeon(R) CPU E5-1650 v3 @ 3.50GHz GenuineIntel GNU/Linux


I do appreciate hints to overcome this situation.

--
Toralf