file system full on a single disk?

[Christian Kujau <lists@nerdbynature.de>]

Hi,

I realize that this comes up every now and then but always for slightly 
more complicated setups, or so I thought:


============================================================
# df -h /
Filesystem             Size  Used Avail Use% Mounted on
/dev/mapper/luks-root  825G  389G     0 100% /

# btrfs filesystem show /
Label: 'root'  uuid: 75a6d93a-5a5c-48e0-a237-007b2e812477
        Total devices 1 FS bytes used 388.00GiB
        devid    1 size 824.40GiB used 395.02GiB path /dev/mapper/luks-root

# blockdev --getsize64 /dev/mapper/luks-root | awk '{print $1/1024^3, "GB"}'
824.398 GB

# btrfs filesystem df /
Data, single: total=388.01GiB, used=387.44GiB
System, single: total=4.00MiB, used=64.00KiB
Metadata, single: total=2.01GiB, used=1.57GiB
GlobalReserve, single: total=512.00MiB, used=80.00KiB
============================================================


This is on a Fedora 31 (5.4.8-200.fc31.x86_64) workstation. Where did the 
other 436 GB go? Or, why are only 395 GB allocated from the 824 GB device?

I'm running a --full-balance now and it's progressing, slowly. I've seen 
tricks on the interwebs to temporarily add a ramdisk, run another balance, 
remove the ramdisk again - but that seems hackish.

Isn't there a way to prevent this from happening? (Apart from better 
monitoring, so I can run the balance at an earlier stage next time).


Thanks,
Christian.


# btrfs filesystem usage -T /
Overall:
    Device size:                 824.40GiB
    Device allocated:            395.02GiB
    Device unallocated:          429.38GiB
    Device missing:                  0.00B
    Used:                        388.00GiB
    Free (estimated):            435.94GiB      (min: 435.94GiB)
    Data ratio:                       1.00
    Metadata ratio:                   1.00
    Global reserve:              512.00MiB      (used: 0.00B)

                         Data      Metadata System              
Id Path                  single    single   single   Unallocated
-- --------------------- --------- -------- -------- -----------
 1 /dev/mapper/luks-root 393.01GiB  2.01GiB  4.00MiB   429.38GiB
-- --------------------- --------- -------- -------- -----------
   Total                 393.01GiB  2.01GiB  4.00MiB   429.38GiB
   Used                  386.45GiB  1.55GiB 64.00KiB            


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