There's a nice discussion going on over at Cloud Use Cases about the difference between private and public clouds. Below is my comment.
I tend to think of cloud computing as a business model underpinned by virtualization. Just as the virtualization layer abstracts the underlying hardware, there's a front-layer that abstracts the cost, sla, workload definition from the underlying computing infrastructure.
We,and many startups, are using EC2 because the low, standard, predictable and variable cost with elastic provisioning of resources in minutes. And this is all accessed on a self-service basis with full control of the life cycle. This change is hugely revolutionary vs what we faced before with dedicated hosting.
Now when I hear customers talking about building a private cloud, generally they mean the ability to deliver all or a subset of the "standard, predictable variable, elastic, self-service, etc" value. So where's the difference between private cloud and public cloud? It is wholly dependent on your role. If you are building the technology,the technologies and issues are very similar. If you are a consumer, you'd like to treat them both the same,
But if you are the owner or operator, the private cloud is very challenging. You need to think of a couple of things, One, what are the standard environments and workload variations I will allow? How do I condense everything we do into a predictable unit cost? And, how do I provide elasticity? There are many other challenges as well, but those three are biggies for your typical IT geek.
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