Wiki

btrfs subvolume list
btrfs subvolume create valami
btrfs subvolume snapshot valami valami-2020-01-02

btrfs subvolume list
# feljegyezni az id-t : pl 123
mount -o subvol=@valami /dev/sdb /mnt
btrfs subvolume delete -c -i 123 /mnt   # -c = commit, -i = identifer

source: https://archive.kernel.org/oldwiki/btrfs.wiki.kernel.org/index.php/FAQ.html#if_your_device_is_large_.28.3E16GiB.29

If the filesystem has allocated (but not used) all of the available space, and the metadata is close to full, then df can show lots of free space, but you may still get out of space errors because there isn't enough metadata available.

To see if this is the case, first look for the amount of allocated space with

# sudo btrfs fi show /dev/device

If this shows the „used” value equal to the „total” value on each device, then everything has been allocated, which is the first condition for this problem.

Secondly, look at the amount of space you have in metadata, as reported by

$ btrfs fi df /mountpoint

If the „used” metadata is close to the „total” value, then that's the second condition for this problem, and you should read on. What does „close” mean? If the free space in metadata is less than or equal to the block reserve value (typically 512 MiB, but might be something else on a particularly small or large filesystem), then it's close to full.

If you have full up metadata, and more than 1 GiB of space free in data, as reported by btrfs fi df, then you should be able to free up some of the data allocation with a partial balance:

# btrfs balance start /mountpoint -dlimit=3

We know this isn't ideal, and there are plans to improve the behavior. Running close to empty is rarely the ideal case, but we can get far closer to full than we do.