Overlay-filesystem (or Unionfs) is a filesystem service that uses union mount to mount different filesystems hierarchies to appear as one unified filesystem. The overlay-filesystem overlays one  filesystem above the other into a layered representation.

When a directory appears in both layers, overlayfs forms a merged directory for both of them. In case of two files have the same name in both layers, only one is served from the  upper or the lower layer, but if a file only exists in the lower layer and an edit needs to be done on that file, a copy of this file is created on the upper layer to be edited.

In most cases the lower layer is normally a read-only filesystem, while the upper layer is read-write one, to allow what is called copy on write, which allows only the writes to the upper filesystem leaving the base lower unchanged.

LXC and Overlayfs

Overlayfs can be used as a backing store for lxc, it can be used with lxc-create and lxc-clone commands.

Note that using overlayfs  with -B option is supported since lxc 1.0. Now lets see how to use overlayfs with lxc.

lxc-create

This command used to create a new lxc container,  choosing a different backing store other than the default simple directory (/var/lib/lxc/container-name/rootfs) is done using -B option, the valid backing stores are dir, lvm, btrfs, overlayfs, and zfs.

To create new container with overlayfs, you can simply write:

[email protected]:~#lxc-create --name lxctest --template ubuntu -B overlayfs

After the container is created, you can see that a new directory has been created within /var/lib/lxc/container-name/ with the name delta0 which represents the upper writable layer of the overlayfs.

[email protected]:~# ls /var/lib/lxc/lxctest/
 config delta0 fstab rootfs

LXCwithOverlayfs

delta0 will store only the changes that has been made to the container, starting the container using lxc-start:

[email protected]:~# lxc-start -n lxctest
[email protected]:~# ls /var/lib/lxc/lxctest/delta0/
dev etc run var

 

By editing any file on the container, a copy of this file will be created on delta0 and then changes will applied on that copy.

[email protected]:~# lxc-info --name lxctest
 Name: lxctest
 State: RUNNING
 PID: 23626
 IP: 10.0.3.208
......
[email protected]:~# ssh [email protected]

[email protected]:~$ sudo echo 'This file will be stored on delta0' > /home/ubuntu/test
[email protected]:~$ exit

[email protected]:~# cat /var/lib/lxc/lxctest/delta0/home/ubuntu/test
This file will be stored on delta0
[email protected]:~# cat /var/lib/lxc/lxctest/rootfs/home/ubuntu/test
cat: /var/lib/lxc/lxctest/rootfs/home/ubuntu/test: No such file or directory

lxc-clone

Clones are two types: copy clone, and snapshot clone. The copy clone is simply a copy from the cloned container, a copy of the  rootfs of the old container is created and assigned to the new container.

A snapshot clone will create a new container with the same rootfs of  the old container, this new container will use the copy on write feature to store the changes made to a different place. We will use overlayfs to create a lxc clone from a “simple directory” container, note that you cant create a snapshot clone from a running container.

[email protected]:~# lxc-clone -o lxctest -n snapclone -B overlayfs -s
 lxc_container: lxccontainer.c: lxcapi_clone: 2733 error: Original container (lxctest) is running
 clone failed
[email protected]:~# lxc-stop --name lxctest
[email protected]:~# lxc-clone -o lxctest -n snapclone -B overlayfs -s

The configuration file will show that the rootfs of the new clone is the same as old container:

[email protected]:~# cat /var/lib/lxc/snapclone/config | grep lxc.rootfs
lxc.rootfs = overlayfs:/var/lib/lxc/lxctest/rootfs:/var/lib/lxc/snapclone/delta0

The snapshot clone is a great method to provision a new custom container for testing and development.