Contributed article from Jeremy Pries, VP of Engineering at Xcedex.

It seems like a good time to bring this up. vSphere is the hot news, and it has the ability to run very large workloads; 8 vCPUs and 256GB of RAM per VM. So we technically can run most workloads as virtual machines, it must be about 99%.

Before I get into it, let me quickly define candidate selection...

Simply the process of looking through the list of servers in a data center (candidates) and using a methodology to pick which ones should be moved to the virtual infrastructure; this could also be for future workloads (not yet deployed workloads). Candidate selection really defines the scope of the virtual infrastructure.

Now, VMware would love it if you virtualized 100% of your x86 environment on their platform. That might happen eventually, but it might not make the most sense right now for you. Or it may not be realistic to get done in the near future. Or it might not make financial sense for you.

So, why would you want to use a method of picking a subset of your datacenter?

1. Low hanging fruit. If you select and prioritize the high consolidation ratio workloads first, you will recover your investment sooner and save more money.
2. Project size. If a project is too big, it will not succeed without being broken into smaller, more manageable chunks. Candidate selection can be a method of breaking large projects into phases that are manageable.
3. Cost. It’s not free to virtualize a workload (even if you are using a free hypervisor). If you run a single vm that is allocated 8 vCPUs and 256GB RAM, I’m going to assume it is using a nice chunk of that capacity. On many servers today (April 2009), that doesn’t leave a lot of room for others. So the point is that it might make financial sense to establish a cut off somewhere.
4. Justification. You can use the selection criteria as a way to justify to your users that the resulting VM is going to perform adequately. In some environments, this is a very political process. Simply having a methodology gains you credibility. Then supporting performance information helps seal the deal.
5. Business Rules. Sometimes business rules are in place that prevent a server from being a virtualization candidate. Maybe the business has a rule that states no MSSQL servers, or no Domain Controllers, or no web servers for the mortgage app, …you get the point, and it doesn’t always make sense. This category is a little different than the other reasons for candidate selection. This is essentially documenting a decision already made instead of making a decision and documenting it.

Somewhere down the road I definitely expect everything to be a VM. For many people I know, every new deployment is a VM. We will get there, but for today there are several different use cases for candidate selection.

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Special thanks to Jeremy Pries, VP of Engineering at Xcedex.  Read more of his thoughts on his personal blog, JeremyPries.com.