Virtualization (LXC Containers)

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As of Fall 2009, we use Linux containers to maintain virtual machines, most notably caffeine, which is hosted on glomag. The various commands to manipulate Linux containers are prefixed with "lxc-"; see their individual manpages for usage.

Management Quick Guide

To manage containers, use the lxc-* tools, which require root privilege. Some examples (replace caffeine with the appropriate container name):

# check if caffeine is running
lxc-info -n caffeine

# start caffeine in the background
lxc-start -d -n caffeine

# stop caffeine gracefully
lxc-halt -n caffeine

# stop caffeine forcefully
lxc-stop -n caffeine

# launch a TTY console for the container
lxc-console -n caffeine

To install Linux container support on a recent Debian (squeeze or newer) system:

  • Install the lxc and bridge-utils packages.
  • Create a bridged network interface (this can be configured in /etc/network/interfaces as though it were a normal Ethernet device, with the additional bridge_ports parameter. This is usually called br0 (can be created manually with brctl). LXC will create a virtual Ethernet device and add it to the bridge when each container starts.

To start caffeine, run the following command as root on glomag:

lxc-start -d -n caffeine

Containers are stored on the host filesystem in /var/lib/lxc (root filesystems are symlinked to the appropriate directory on /vm).

ehashman's Guide to LXC on Debian

Configuring the host machine

First, install all required packages:

# apt-get install lxc bridge-utils

Setting up ethernet bridging

Next, create an ethernet bridge for the container. Edit /etc/network/interfaces:

# The primary network interface
#auto eth0
#iface eth0 inet static
#       address
#       netmask
#       gateway

# Bridge ethernet for containers
auto br0
iface br0 inet static
    bridge_ports eth0

Cross your fingers and restart networking for your configuration to take effect!

# ifdown br0 && ifup br0
// bash enter to see if you lost connectivity and have to make a machine room trip

Note: !!! Do not use !!!

# service networking restart

The init scripts are broken and this likely will result in a machine room trip (or IPMI power cycle).

Setting up storage

Last, allocate some space in your volume group to put the container root on:

// Find the correct volume group to put the container on
# vgdisplay

// Create the volume in the appropriate volume group
# lvcreate -L 20G -n container vg0

// Find it in the dev mapper
# ls /dev/mapper/

// Create a filesystem on it
# mkfs.ext4 /dev/mapper/vg0-container

// Add a mount point
# mkdir /vm/container 

Last, add it to /etc/fstab:

/dev/mapper/vg0-container /vm/container        ext4    defaults        0       2

Test the entry with mount:

# mount /vm/container

Now you're done!

Creating a new container

Create a new container using lxc-create:

// Create new container "container" with root fs located at /vm/container
# lxc-create --dir=/vm/container -n container --template download

This will prompt you for distribution, release, and architecture. (Architecture must match host machine.)

Take this time to review its config in /var/lib/lxc/container/config, and tell it to auto-start if you like:

# Auto-start the container on boot = 1

You'll also want to set up networking (if applicable):

# Networking = veth = up = br0 = eth0 = de:ad:be:ef:ba:be  # or something sensible


// List containers, -f for fancy
# lxc-ls -f

to ensure that your container has been successfully created; it should be listed. You can also list its root directory if you like. To start it in the background and obtain a root shell, do

// Start and attach a root shell
# lxc-start -d -n container
# lxc-attach -n container

Migrating a container between hosts

Start by shutting the container down:

root@container:~# halt

Then make a tarball of the container's filesystem:

# tar --numeric-owner -czvf container.tar.gz /vm/container

Copy it to its target destination, along with the configs:

$ scp container.tar.gz new-host:
$ scp -r /var/lib/lxc/container/ new-host:/var/lib/lxc/

Now carefully extract it. If you haven't already, provision storage and ethernet per the container creation section.

Yes, we really do want to stick it directly into /:

# tar --numeric-owner -xzvf container.tar.gz -C /

Verify the container's existence:

# lxc-ls -f
container  STOPPED  -     -     YES   

Now just start it on up:

# lxc-start -d -n container

And test by trying an ssh in!