Configuring for LXC


The LXC local provider enables multiple Juju-deployed services with a single operating system. This easily makes available a number options: evaluation of Juju; evaluation of the software being deployed, experimentation with various service configurations; and charm development.

Begin by adding the Juju stable release PPA:

sudo apt-add-repository ppa:juju/stable
sudo apt-get update

You can then install the local provider, the commands depend on the Juju version you are using:

For Ubuntu versions newer than 12.04:

sudo apt-get install juju-local

For 12.04 LTS users only:

Because the provider requires a more recent version of LXC, and thus the kernel, that was originally shipped with 12.04, we will install kernel 3.13 from the LTS Hardware Enablement Stack:

sudo apt-get install juju-local linux-image-generic-lts-trusty linux-headers-generic-lts-trusty

You will need to reboot into the new kernel in order to use Juju with the local provider.

If you're not running Ubuntu please consult your operating system distribution's documentation for instructions on installing the LXC userspace tools and the MongoDB server. Juju requires a MongoDB server built with SSL support.


Start by generating a generic configuration file for Juju and then switching to the local provider:

juju generate-config
juju switch local

This will generate a file, environments.yaml (if it doesn't already exist), which will live, on Ubuntu, in your ~/.juju/ directory (and will create the directory if it doesn't already exist).

Note: If you have an existing configuration, you can use juju generate-config --show to output the new config file, then copy and paste relevant areas in a text editor etc.

The generic configuration sections generated for the local provider will look something like this and will normally work out of the box (no need to edit):

    type: local
    admin-secret: 772b9471131c6b5883475e3908156d32
    # Override the directory that is used for the storage files and database.
    # The default location is $JUJU_HOME.
    # $JUJU_HOME defaults to ~/.juju
    # root-dir: ~/.juju/local
    # Override the storage port if you have multiple local providers, or if the
    # default port is used by another program.
    # storage-port: 8040
    # Override the shared storage port if you have multiple local providers,
    # or if the default port is used by another program.
    # shared-storage-port: 8041

Using Juju with this configuration, the storage files and the database will be located in the directory specified by the environment variable $JUJU_HOME, which defaults to ~/.juju/. By uncommenting and setting root-dir this location can be changed as well as the ports of the storage and the shared storage. This may be useful if you want to run multiple local providers simultaneously or to deal with possible conflicts with other programs on your system.

Ensure all local providers are using different ports. The default port numbers for api-port, state-port, and storage-port are 17071, 37017, and 8040 respectively. Also, the name of each provider is arbitrary. For instance:

# another local environment
    type: local
    api-port: 17072
    state-port: 37018
    storage-port: 8041

Note: If your home directory is encrypted you cannot point $JUJU_HOME or root-dir to a location within it. Use locations outside of it.

Bootstrapping and Destroying

The usage of LXC containers requires root privileges for some steps and Juju will prompt for your password if needed. Juju cannot be run under sudo because it needs to manage permission as the real user.

Note: If you are running iptables (firewall) or even an iptables frontend such as ufw, the LXC local provider might not work properly. Troubleshoot accordingly or stop the firewall altogether.

If you have used the local provider in the past when it required sudo, you may need to manually clean up some files that are still owned by root. If your local environment is named "local" then there may be a local.jenv owned by root in the JUJU_HOME directory (~/.juju). After the local environment is destroyed, you can remove the file like this:

sudo rm ~/.juju/environments/local.jenv

Fast LXC creation

The local provider can use lxc-clone to create the containers used as machines. This feature is controlled by the lxc-clone option in environments.yaml. The default is "true" for Trusty and above, and "false" for earlier Ubuntu releases.

You can try to use lxc-clone on earlier releases, and it may well work, but it is not a supported feature. You can enable lxc-clone in environments.yaml like this:

    type: local
    lxc-clone: true

The local provider is btrfs-aware. If your LXC directory is on a btrfs filesystem, the clones use btrfs snapshots and are much faster to create and take up much less space. There is also support for using aufs as a backing-store for the LXC clones, but there are some situations where aufs doesn’t entirely behave as intuitively as one might expect, so this must be turned on explicitly in environments.yaml.

    type: local
    lxc-clone-aufs: true

When using clone, the first machine to be created will create a "template" machine that is used as the basis for the clones. This will be called juju-<series>-template, so for a precise image, the name is juju-precise-template. Do not modify or start this image while a local provider environment is running because you cannot clone a running lxc machine.

Newly provisioned machines on the Local Provider have package upgrades disabled by default. This, again, is to accelerate provisioning. To allow automatic software upgrades to occur you will need to configure accordingly. See General config options.

Juju caching for LXC images

Starting with Juju 1.22, the first time a host (local or remote) needs a LXC image it will be downloaded from and cached on the state server (MongoDB). The same image will be copied to the host's filesystem (/var/cache/lxc) if LXC host caching is enabled (the default).

This means that the external retrieval of images is done once per environment, and not once per machine which is the normal behaviour for LXC.

Use of cached images

Once the Juju LXC cache, and optionally the LXC host cache, is populated:

  • Juju will supply its cached image to a new (non-local) machine that needs it for its own containers.
  • When Juju creates a (local) container, that container will use the LXC host cached image (if enabled). Otherwise it will use the Juju cached image.

General notes on Juju image caching

  • It applies to all provider types.
  • It is independent of image cloning ('lxc-clone'), which is enabled by default.
  • Future development work will allow Juju to automatically download new LXC images when they become available.
  • It is only available to hosts installed with 1.22 (or greater); not upgraded to that level.

Recall that LXC containers can be deployed on all types of providers, not just the Local Provider.


The 'cached-images' command can list and delete cached LXC images stored in the Juju environment. The 'list' and 'delete' subcommands support '--arch' and '--series' options to filter the result.


To see all cached images:

juju cached-images list

To see just the amd64 trusty image:

juju cached-images list --series trusty --arch amd64

To delete the amd64 trusty image:

juju cached-images delete --kind lxc --series trusty --arch amd64

See 'juju cached-images list --help' and 'juju cached-images delete --help' for more details.


Juju cached LXC images do not return to the cache once deleted (due to host caching). See LP bug #1483987.

Ensuring a fresh cache

Due to the interaction of both the Juju cache and the LXC host cache, in addition to the above issue, to ensure stale images are not being used it is recommended to simply flush both caches together and on a regular basis:

For the Juju cache, as shown previously:

juju cached-images delete --kind lxc --series trusty --arch amd64

For the LXC host cache:

sudo rm -r /var/cache/lxc/cloud-trusty

Do not forget to also remove the source clone image (template) if lxc-clone is enabled (the default):

sudo lxc-destroy -n juju-trusty-lxc-template

LXC Containers within KVM guests

You can also use Juju to create KVM guests within which are placed LXC containers. See Configuring for KVM.