repeated enospc errors during balance on a filesystem with spare room - pls advise

[Ole Langbehn <neurolabs.de@gmail.com>]

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Hi,

I have done three full balances in a row, each of them ending with an
error, telling me:

BTRFS info (device nvme1n1p1): 2 enospc errors during balance
BTRFS info (device nvme1n1p1): balance: ended with status: -28

(first balance run it was 4 enospc errors).

The filesystem has enough space to spare, though:

# btrfs fi show /
Label: none  uuid: 34ea0387-af9a-43b3-b7cc-7bdf7b37b8f1
        Total devices 1 FS bytes used 624.36GiB
        devid    1 size 931.51GiB used 627.03GiB path /dev/nvme1n1p1

# btrfs fi df /
Data, single: total=614.00GiB, used=613.72GiB
System, single: total=32.00MiB, used=112.00KiB
Metadata, single: total=13.00GiB, used=10.64GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

This is after the balances, but was about the same before the balances.
Before them, data had about 50GB diff between total and used.

The volume contains subvolumes (/ and /home) and snapshots (around 20
per subvolume, 40 total, oldest 1 month old).

My questions are:

1. why do I get enospc errors on a device that has enough spare space?
2. is this bad and if yes, how can I fix it?



A little more (noteworthy) context, if you're interested:

The reason I started the first balance was that a df on the filesystem
showed 0% free space:

# df
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/nvme1n1p1 976760584 655217424 	   0 100% /
...

and a big download (chromium sources) was aborted due to "not enough
space on device".

I monitored the first balance more closely, and right after the start,
df looked normal again, showing available blocks, but during the
balance, it flip-flopped a couple of times between again showing 0
available bytes and showing the complement between actual size and used
bytes. I did not observe this behavior any more during balance 2 and 3,
but did not observe as closely.

TiA for any insights and ideas on how to proceed and a healthy start
into the new year for everyone.





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