Current versions of Ubuntu and Kubuntu install a package named apt-btrfs-snapshot by default. This package creates a btrfs snapshot each time packages are installed or removed. These snapshots can take up a lot of space if not cleaned up regularly.

How much space is left on my btrfs file system?

First a quick reminder. Traditional Linux tools like df might report the used / free disk space on btrfs file systems inaccurately. Better use the following commands to get an accurate overview about your current disk space situation:

user@laptop:~/tmp$ sudo btrfs filesystem usage /

Overall:
    Device size:                 156.57GiB
    Device allocated:            126.16GiB
    Device unallocated:           30.41GiB
    Device missing:                  0.00B
    Used:                         37.35GiB
    Free (estimated):            113.61GiB      (min: 113.61GiB)
    Data ratio:                       1.00
    Metadata ratio:                   1.00
    Global reserve:               93.14MiB      (used: 0.00B)
    Multiple profiles:                  no

Data,single: Size:119.65GiB, Used:36.44GiB (30.46%)
   /dev/sdb6      57.65GiB
   /dev/sda5      62.00GiB

Metadata,single: Size:6.51GiB, Used:926.61MiB (13.90%)
   /dev/sdb6       2.51GiB
   /dev/sda5       4.00GiB

System,single: Size:4.00MiB, Used:48.00KiB (1.17%)
   /dev/sdb6       4.00MiB

Unallocated:
   /dev/sdb6       3.63GiB
   /dev/sda5      26.77GiB

user@laptop:~/tmp$

If you just want to see which partitions belong to a btrfs file system the following command comes in handy:

user@laptop:~/tmp$ sudo btrfs dev usage /

/dev/sdb6, ID: 1
   Device size:            63.79GiB
   Device slack:              0.00B
   Data,single:            57.65GiB
   Metadata,single:         2.51GiB
   System,single:           4.00MiB
   Unallocated:             3.63GiB

/dev/sda5, ID: 2
   Device size:            92.77GiB
   Device slack:              0.00B
   Data,single:            62.00GiB
   Metadata,single:         4.00GiB
   Unallocated:            26.77GiB

user@laptop:~/tmp$

Removing btrfs snapshots (created by apt-btrfs-snapshot)

To list all the snapshots in a btrfs file system the following command can be used: $ sudo btrfs subvolume list -s /. Using -s only snapshot subvolumes in the filesystem will be listed. A snapshot is also a subvolume, but with a given initial content of the original subvolume. If you want to read more about btrfs snapshots and subvolumes see here.

But instead of messing around with the raw snapshots / subvolumes I recommend using the apt-btrfs-snapshot tool itself instead.

Execute $ sudo apt-btrfs-snapshot list to list all btrfs snapshots created by apt-btrfs-snapshot. To remove all the snapshots you can use the following command: $ sudo apt-btrfs-snapshot delete-older-than 0d.

Check your file system

After the snapshots have been removed you might want to run a file system check:

# Start a file system check.
$ sudo btrfs scrub start /

# Check the status of a running file system check.
$ sudo btrfs scrub status /

If you want to get to know more about some of the btrfs terms you might take a look at the glossary over at the btrfs wiki. A scrub is defined as follows:

An online filesystem checking tool. Reads all the data and metadata on the filesystem, and uses checksums and the duplicate copies from RAID storage to identify and repair any corrupt data.