Add a KVM host


#1

After installing MAAS, the ‘Pods’ page is typically empty:

2.5+

Once a machine as been added to MAAS and gone through enlistment, commissioning and hardware testing, you can deploy it (after acquiring it) as a KVM host by ticking the ‘Install MAAS-managed KVM Host’ checkbox:

CLI

Once a machine has been enlisted, commissioned, and acquired, you can tell MAAS to deploy it as a KVM host:

maas $PROFILE machine deploy <system_id> install_kvm=True

Manual/Pre-2.5

Setting up a manual KVM host in 2.5 or on an older version of MAAS requires more steps.

Set up a `maas` libvirt network

Libvirt by default creates a virtual bridge, virbr0, through which VMs communicate with each other and the Internet. DHCP is supplied by libvirt so that new VMs are automatically given an IP address.

However, to enable network booting in MAAS, you’ll need to provide DHCP in MAAS and either:

  1. Disable DHCP on libvirt’s default network, or
  2. Create a new libvirt network maas with DHCP disabled.

You can set up such a maas network like this:

cat << EOF > maas.xml
<network>
 <name>maas</name>
 <forward mode='nat'>
   <nat>
     <port start='1024' end='65535'/>
   </nat>
 </forward>
 <dns enable="no" />
 <bridge name='virbr1' stp='off' delay='0'/>
 <domain name='testnet'/>
 <ip address='172.16.99.1' netmask='255.255.255.0'>
 </ip>
</network>
EOF
virsh net-define maas.xml

Note that this network also has NAT port forwarding enabled to allow VMs to communicate with the Internet at large. This is useful in test environments.

Set up SSH

In order for MAAS to successfully communicate with libvirt on your KVM host machine, this example command must succeed from every rack controller as user maas:

virsh -c qemu+ssh://$USER@$KVM_HOST/system list --all

Here, $USER is a user on your KVM host who is a member of the libvirtd unix group on the KVM host, and $KVM_HOST is the IP of your KVM host.

The maas user on your rack controllers will issue all virsh commands. Therefore, you’ll need to set up SSH public keys on every rack controller for user maas.

To do this, first create SSH keys on all rack controllers:

sudo chsh -s /bin/bash maas
sudo su - maas
ssh-keygen -t rsa -N ''

Next, add the contents of ~maas/.ssh/id_rsa.pub to the KVM host user’s ~$USER/.ssh/authorized_keys. To do this, you can, for example, log into your KVM host node via SSH from a host for which you provided MAAS an existing public SSH key (e.g. your imported Launchpad keys).

Insufficient permissions for $USER may cause the virsh command to fail with an error such as failed to connect to the hypervisor. Check the $USER group membership to make sure $USER is a member of the libvirtd group.

Add

Now, add a KVM host by using the ‘Add pod’ button. Choose ‘Virsh (Virtual systems)’ from the ‘Pod type’ drop-down menu.

Here, ‘Virsh address’ typically looks like the following:

qemu+ssh://<kvm host IP>/system

MAAS will automatically discover and store the resources your KVM host contains. Any existing machines will also appear on the ‘Machines’ page and be commissioned.

Add with CLI

Create a KVM host:

maas $PROFILE pods create type=virsh power_address=qemu+ssh://ubuntu@192.168.1.2/system

Create a KVM host with overcommitted resources:

maas $PROFILE pods create type=virsh power_address=qemu+ssh://ubuntu@192.168.1.2/system \
        power_pass=example cpu_over_commit_ratio=0.3 memory_over_commit_ratio=4.6

Create a KVM host that uses a default storage pool:

maas $PROFILE pods create type=virsh power_address=qemu+ssh://ubuntu@192.168.1.2/system \
        power_pass=example default_storage_pool=pool1

Configuration

KVM hosts have several configuration options. Modify these by selecting the ‘Configuration’ tab and clicking ‘Edit’. Options include a KVM host’s location, password, network zone, and default resource pool.

Overcommit resources

Overcommitted resources are those allocated beyond what’s available in the physical resource. Sliders on the configuration page allow you to strictly limit whether CPU and memory can be over committed, and to what extent. The input fields to the right of the sliders accept floating point values from 0 to 10, with a default value of 1.

The following shows theoretical examples of these ratios and how they affect physical resource allocation:

  • 8 physical CPU cores * 1 multiplier = 8 virtual CPU cores
  • 8 physical CPU cores * 0.5 multiplier = 4 virtual CPU cores
  • 32 physical CPU cores * 10.0 multiplier = 320 virtual CPU cores
  • 128GB physical Memory * 5.5 multiplier = 704G virtual Memory

Overcommitting resources allows a user to compose many MAAS-managed machines without worrying about the physical limitations of the host. For example, on a physical host with 4 cores and 12 GB of memory, you could compose 4 virsh nodes, each using 2 cores and 4 GB of memory, obviously over-committing the available physical resources. Provided you never run all 4 simultaneously, you’d have all the benefits of MAAS-managed VMs without over-taxing your host.


Documentation index
MAAS Documentation
KVM introduction
KVM host networking