mjhjmtu

My Kubuntu 12.04 system ran out of space on on the root partition and will not boot. The command df -h shows a lot of space available (with only 37% used):

/dev/sda2 45G 17G 29G 37%

The following page indicates that I should run the balance command:

https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#I_get_.22No_space_left_on_device.22_errors.2C_but_df_says_I.27ve_got_lots_of_space

$ sudo btrfs fi balance start -dusage=5 /mount/point

I'm not entirely confident that this is the best approach, but it is the only one I found. However, when I run that command, I get this error:

ERROR: error during balancing '/blah/blah/blah' - No space left on device

I get the same error with:

$ sudo btrfs fi balance start -dusage=1 /mount/point

What is the right solution?

There are ways to get balance to run in this situation.

sudo btrfs fi show
sudo btrfs fi df /mount/point
sudo btrfs fi balance start -dusage=10 /mount/point

If the balance command ends with "Done, had to relocate 0 out of XX chunks", then you need to increase the "dusage" percentage parameter till at least one chunk is relocated.

if the balance command fails with:

ERROR: error during balancing '/blah/blah/blah' - No space left on device

You might actually need to delete files from the device to make some room. Then run the balance command again.

However, thanks to Marc's Blog: btrfs - Fixing Btrfs Filesystem Full Problems here is another option:

One trick to get around this is to add a device (even a USB key will
do) to your btrfs filesystem. This should allow balance to start, and
then you can remove the device with btrfs device delete when the
balance is finished. It's also been said on the list that kernel 3.14
can fix some balancing issues that older kernels can't, so give that a
shot if your kernel is old.

I tried everything in the accepted answer, and Marc's blog, including incrementing the -dusage parameter and adding another block device, all to no avail. Even after deleting some files and freeing up a little space on the disk which was full, balance was not able to complete. For some reason it always seemed to be moving data onto the nearly full disk. In the end what did work for me was restricting balancing to the full device:

btrfs balance start -ddevid=<dev_id> <path>

where the dev_id can be found with:

btrfs fi show