Adding more control nodes adds more complexity than adding compute nodes. Generally, when additional control nodes are added, each of the control services need to be scaled and the database and message bus need to become highly available.

Triple-O has the capability to install and configure highly available scaled services for you. There is a wealth of documentation on OpenStack's Triple-O Wiki at http://docs.openstack.org/developer/tripleo-docs/. Be sure to take a look at the documentation there if you are trying to deploy something that has not been covered in this book.

As long as there are bare-metal nodes available, just a few extra parameters need to be added to the overcloud deploy command to deploy a multi-control node deployment. It is not recommended to update an existing control tier. It is recommended to plan ahead accordingly and deploy all the control nodes for the first using these parameters. Here is an example of a deployment that would have three control nodes, three compute nodes and three ceph nodes. Three control nodes is the minimum number of control nodes in a multi-control node deployment. More information on why three control nodes is the minimum in the High availability section coming up:

undercloud# openstack overcloud deploy --control-scale 3 --compute-scale 3 --templates --libvirt-type qemu --ceph-storage-scale 3 -e /usr/share/openstack-tripleo-heat-templates/environments/storage-environment.yaml -e /usr/share/openstack-tripleo-heat-templates/environments/puppet-pacemaker.yaml --ntp-server pool.ntp.org

The addition to this command includes the increased number of each type of node, the puppet-pacemaker.yaml file and the ntp-server. Let's take a brief look in more detail at what will be added by the additional configuration parameters.