What is undercloud and what is overcloud?

OpenStack-on-OpenStack or TripleO is an official OpenStack project for deployment of OpenStack systems. It is often asked by the new users – what is undercloud and what is overcloud? As can be understandable, all of these phrases are related to OpenStack Cloud Computing Software Development and HP Helion Cloud. HP is the single largest code contributor to OpenStack and there is HP Helion OpenStack as product, the unbranded phrases got in to the OpenStack project.

What is undercloud and what is overcloud in short?

Practically there are two ways to fully use OpenStack for dev purpose on real datacenter unless you have own datacenter – via HP and or directly via OpenStack. Definitely the software part is Free Software in both cases. HP Helion has an option on Horizon to boot their material as an instance from image – this thing is absent on Rackspace. HP Helion OpenStack is a commercial distribution built on OpenStack technology but the software bears a Free Software License, that is why there is a HP Helion OpenStack Community Edition. Recall the “Freedom” phrase said by Richard Stallman for Free Software. HP is a bigger sponsor of Free Software Foundation. Free Software is not about the cost. $1,200/year/server is the cost of HP Helion OpenStack. The human support and their technologies are the differentiating points.

Practically, you need to run the Software on some hardware, HP Helion OpenStack is a bit tailored for the real hardware. HP manufactures the servers plus they have different types of clients. HP needs no other certification, they are the server manufacturers. Rackspace OpenStack Community Edition does not exist, instead the have Rackspace Private Cloud Software, which is also a Free Software. HP Helion OpenStack also allows to deploy a private cloud.

To strip off the branding, the TripleO cloud separately runs as a part OpenStack which is operated by the TripleO CD admins team. Most documents about undercloud and overcloud will match with either RedHat or HP. There are code contributors from RedHat for TripleO cloud on OpenStack.

First one need to have a seed first. A seed virtual machines is a bootable KVM virtual machines image provided as part of HP Helion OpenStack and is deployed in a VM instance. This image contains the minimum services required of OpenStack to function and to be able to provision physical hardware to deploy the undercloud. The undercloud server is a basic single-node OpenStack installation running on a single physical server used to deploy, test, manage, and update the overcloud servers. There is no HA configuration for the undercloud.

The overcloud is the functional cloud available to run guest virtual machines and workloads. It offers access to the Nova, Neutron, Glance, Keystone, Cinder, Swift, Ironic, Heat, Horizon, and Ceilometer services. API access to the undercloud is available to tripleo-cd-admins.