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.

For me the answer is always "snapshots" and normally because of docker.

If you run a docker image store on a BTRFS drive, docker creates snapshots at various times. It never cleans them up; It has no commands that clean them up, and it means that if you delete a file it doesn't free any space because the snapshots keep the file alive.

also, 'df -i'. probably not this case but...

Looking at balancing might be right place to start. ref, https://archive.kernel.org/oldwiki/btrfs.wiki.kernel.org/index.php/FAQ.html#Help.21_I_ran_out_of_disk_space.21

You might want to start by rebalancing by percentages and not all at once. If nothing else it'll tell you much sooner if you're on the right track or not. Something like sudo btrfs balance start -dusage=20 -musage=20 /mnt/disk3 to work on only blocks that are 20% full or less. That should coaleace them into single data blocks and free up some others.

As a rule of thumb you should keep your disk usage around 60% or under.

My guess it that you have snapshots or other similar hidden data taking up space. List out your snapshots and sub volumes.

The metadata seems to be pretty close to full. I'm not using BTRFS, but I've read earlier that this is solved with rebalancing, or how is that called. Btrfs's management command is able to do it.

But possibly the metadata is unreasonably large, so maybe the solution is not rebalancing.

Have you tried a rebalance? What's up over there?

Would be nice if there's some automatic solution, but after running into this issue I always run a couple different btrfs balance after deleting larger files for good measure. Took a while to figure out why Linux said there wasn't any space left when df reported several GB available on the root partition