Many view virtualization as a technical challenge, and focus on determining how many applications can be combined together on as little hardware as possible. I would challenge this notion, and say that it is instead a business challenge, which largely revolves around determining which applications should be placed together from a business perspective.
The difference is important, and any confusion between the two can be explained quite simply. Many virtualization projects start out in the lab, which is relatively free of business constraints. This tends to cause early virtualization investigations to focus on the technical aspects of virtualization, and the criteria used to combine applications is almost universally driven by workload considerations.
Once you go beyond the lab, however, the "real world" production environments are riddled with constraints, including change restrictions, availability commitments, business owner relationships, compliance rules, etc.. To charge into such environments using workload as the primary consideration for virtualization is to run the risk of constructing a completely dysfunctional infrastructure. Consider a simple case: by combining applications that have non-overlapping maintenance windows onto a single physical server you may never be able to do hardware maintenance again.
This is but one example, and to list all the business constraints that govern production environments would be daunting. But for a start I always recommend incuding the following basic constraints when analyzing production environments:
- physical location
- operational environment
- application tier
- availabilty target
There are many others, but these are a good start. It is rare that one would seek to combine a production server and its DR counterpart on the same server, and combining servers from different application tiers on the same system can create correlated workloads that slow transaction response times. Location is also usually a big constraint, although this is often lifted if the virtualization is part of a larger data center consolidation. Keeping availability levels consistent between VMs is always wise, and allows you to size and spec hardware appropriately, particularly wrt redundancy levels and UPS requirements.
These are specific examples, but are part of a general trend. As virtualization moves out of the lab and into production, the challenge becomes less and less about the the specific virtualization technology and how to stack workloads on it, and more about the target environment and the proper consideration of all the constraints within it.
