How to figure out why BTRFS drive reports it is out of space

daq@lemmy.sdf.org to Linux@lemmy.ml – 30 points – 
# sudo btrfs fi df /mnt/disk3
Data, single: total=12.70TiB, used=12.27TiB
System, DUP: total=8.00MiB, used=1.34MiB
Metadata, DUP: total=15.00GiB, used=14.50GiB
GlobalReserve, single: total=512.00MiB, used=608.00KiB

# mkdir /mnt/disk3/tst
mkdir: cannot create directory ‘tst’: No space left on device

I suspect this is BTRFS balancing issue, but even BTRFS's own utility is indicating there's still SOME space left. Certainly should be enough to create a directory.

Any ideas?

Just in general BTRFS default options for creating new volumes seem to not work well for disks that I intend to fill completely immediately after formatting. Are there better options for this use case? I just use

# mkfs.btrfs /dev/sdd1

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When you create a filesystem, there is a parameter named as "block percent free". This parameter should be "5%", so a 5% of your partition size can only be written by the "root" user.

You can decrease this value or just free some space. You can try to create files or folders as root as well.

Is there any reason this 5% number still holds true? Back in the days of 40 MB hard drives it made sense to make sure the system didn’t totally run out while root was fixing the low disk situation … but these days even 1% is still several gigabytes of space, not likely to run out that quickly.

Are you sure that's the case with btrfs? I know ext has that feature. My understanding is btrfs just has a global reserve that can be used for any data in an low space situation.

# sudo btrfs fi usage /mnt/disk3
Overall:
    Device size:                  12.73TiB
    Device allocated:             12.73TiB
    Device unallocated:            1.00MiB
    Device missing:                  0.00B
    Device slack:                    0.00B
    Used:                         12.29TiB
    Free (estimated):            449.43GiB      (min: 449.43GiB)
    Free (statfs, df):           449.43GiB
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)
    Multiple profiles:                  no

Data,single: Size:12.70TiB, Used:12.26TiB (96.55%)
   /dev/sdd1      12.70TiB

Metadata,DUP: Size:15.00GiB, Used:14.49GiB (96.58%)
   /dev/sdd1      30.00GiB

System,DUP: Size:8.00MiB, Used:1.34MiB (16.80%)
   /dev/sdd1      16.00MiB

Unallocated:
   /dev/sdd1       1.00MiB