We all know what virtualisation consists of: a host containing one or more virtual machines, usually, but not necessarily, servers.
Scale that up and you get dozens, or hundreds, of servers powering hundreds, if not thousands, of virtual machines. That is becoming typical of today's data centre.
So what is a private cloud in this context? The answer is not so clear cut. Some argue that it is a set of computing resources – networking, storage and raw compute – that an organisation can draw on at will.
Pick your own
It lives either in your data centre or in a co-located facility. The essential characteristic is that you own it and control it. And because you are managing all of it, you pay for all of it.
The difference between virtualisation and private cloud is that one provides the means to achieve the other – but it is not quite so simple.
If you virtualise, you reduce hardware maintenance but you still have to manage those machines, patch their operating systems and applications, and generally keep them up to date.
And when their requirements change – maybe the accounts department is doing its monthly invoice run and needs more disk space and CPU – you have to manage that process manually.
Do the automation
What a private cloud brings is automation of that resource allocation as demand moves around the data centre.
It means the commissioning and de-commissioning of machines and the automatic management of available resources, as well as upgrades, patches and maintenance. Virtual machines can commission additional resources as they need them. In other words, the focus of attention shifts from the infrastructure to service delivery.
If a virtual machine is to have freedom to move, it is not just the server configuration that changes
The thing to remember is that if a virtual machine is to have complete freedom to move around the data centre, it is not just the server configuration that changes: the storage, network policies and security polices need to move too.
There are several other products out there offering to take control of your data centre and help you manage it as a single entity. They include software from Cloud.com, Abiquo and Nimbula, among others.
Nimbula Director promises to help you migrate applications into the cloud. It includes policy-based authorisation, together with dynamic storage provisioning, monitoring and metering, automated deployment and cloud management.
Cloud.com is open source and promises similar benefits. It comes as a free, community-supported edition as well as in a fully supported commercial version. Abiquo also offers community and commercial versions of its software.
But as Mark Bowker, an analyst at the Enterprise Strategy Group, has pointed out, none of the vendors, including the major ones such as VMware, Citrix and Microsoft, are clear as to what a private cloud is, although they are united in their view of virtualisation technology as its bedrock.
To summarise, the key definition of a cloud, whether private or public, is its intelligence and responsiveness to demands from a virtual machine for resources.
But that is just a vision for most companies right now and is likely to remain so unless you buy all your data centre gear from one vendor. Unsurprisingly, that is what the big beasts in the IT jungle would prefer and an option they are all offering.
All they have to do is convince enterprises that it is safe to go back into the single-vendor pool, and that might not be quite so easy. ®