Working with snapshot clones of containers stored on Btrfs filesystem

1. To clone a container, we use lxc-clone command. The option -B btrfs creates a Btrfs volume which later can be used for snapshot clones. Snapshot clones are similar to linked clones of Virtualbox.

# lxc-clone -B btrfs c1 c2

2. We can use -s option to create a snapshot volume.

# lxc-clone -s -B btrfs  c2 c3

Here are some useful Btrfs commands to get more information about underlying volumes of Btrfs.

1. Listing subvolumes

# btrfs subvolume list /vm
ID 257 gen 187 top level 5 path vm
ID 260 gen 168 top level 257 path c2/rootfs
ID 261 gen 168 top level 257 path c3/rootfs

2. Getting details of a subvolume

# btrfs subvolume show /vm/c2/rootfs
/vm/c2/rootfs
    Name:                  rootfs
    uuid:                  68628679-d0ae-724a-be01-5ba74e2c00b1
    Parent uuid:           -
    Creation time:         2015-11-24 17:34:33
    Object ID:             260
    Generation (Gen):      168
    Gen at creation:       161
    Parent:                257
    Top Level:             257
    Flags:                 -
    Snapshot(s):
                           vm/c3/rootfs

3. Enable quotas and see disk usage of subvolumes

# btrfs quota enable /vm

# btrfs qgroup show  /vm
qgroupid rfer      excl      
-------- ----      ----      
0/5      16384     16384     
0/257    827707392 827707392 
0/260    825425920 7421952   
0/261    825425920 7421952   
0/263    825425920 7421952

Here rfer column shows total bytes of data referred by subvolume, and excl column shows bytes of data exclusive for the volume that is not shared with other volumes.