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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.
Fragmentation probably but seems arbitrary
You/I learn something new every day. Cool info!
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
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.
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.
also, 'df -i'. probably not this case but...
btrfs dynamically allocates inodes.
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.
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
I am surprised there isn't an automatic mechanism to handle this especially if it is such a frequent issue.
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.