Understanding "Used" in df

Martin Steigerwald
2020-07-22T15:18:15Z

I have:

% LANG=en df -hT /home
Filesystem Type Size Used Avail Use% Mounted on
/dev/mapper/sata-home btrfs 300G 175G 123G 59% /home

And:

merkaba:~> btrfs fi sh /home
Label: 'home' uuid: […]
Total devices 2 FS bytes used 173.91GiB
devid 1 size 300.00GiB used 223.03GiB path /dev/mapper/sata-home
devid 2 size 300.00GiB used 223.03GiB path /dev/mapper/msata-home

merkaba:~> btrfs fi df /home
Data, RAID1: total=218.00GiB, used=171.98GiB
System, RAID1: total=32.00MiB, used=64.00KiB
Metadata, RAID1: total=5.00GiB, used=1.94GiB
GlobalReserve, single: total=490.48MiB, used=0.00B

As well as:

merkaba:~> btrfs fi usage -T /home
Overall:
Device size: 600.00GiB
Device allocated: 446.06GiB
Device unallocated: 153.94GiB
Device missing: 0.00B
Used: 347.82GiB
Free (estimated): 123.00GiB (min: 123.00GiB)
Data ratio: 2.00
Metadata ratio: 2.00
Global reserve: 490.45MiB (used: 0.00B)
Multiple profiles: no

Data Metadata System
Id Path RAID1 RAID1 RAID1 Unallocated
-- ---------------------- --------- -------- -------- -----------
1 /dev/mapper/sata-home 218.00GiB 5.00GiB 32.00MiB 76.97GiB
2 /dev/mapper/msata-home 218.00GiB 5.00GiB 32.00MiB 76.97GiB
-- ---------------------- --------- -------- -------- -----------
Total 218.00GiB 5.00GiB 32.00MiB 153.94GiB
Used 171.97GiB 1.94GiB 64.00KiB

I think I understand all of it, including just 123G instead of
300 - 175 = 125 GiB "Avail" in df -hT.

But why 175 GiB "Used" in 'df -hT' when just 173.91GiB (see 'btrfs fi sh')
is allocated *within* the block group / chunks?

Does this have something to do with that global reserve thing?

Thank you,
--
Martin