3. Test for Application Stability
King says companies should also conduct rigorous testing on their virtualized mission-critical applications before and after deployment. "You have to make sure that in terms of memory and network bandwidth, each application is stable at all times. For instance, if you know an application is harder-hit during certain times of the year, you'll want to account for that in establishing your ratios," he says.
Testing will also help IT teams determine which virtual workloads will coexist best on a physical host. "You have to make sure that a physical server isn't running multiple VMs with the same workload. Otherwise, if they're all Web servers, they will be contending for the same resources at the same time, and that will hinder your consolidation ratio," says Nelson Ruest, co-author of Virtualization: A Beginner's Guide and founder of Resolutions Enterprises, a consultancy in Victoria, British Columbia. Instead, IT staffers should make sure that workloads are heterogeneous and well balanced based on peak usage times and resource demands.
Ruest also warns IT teams not to forget the spare resources that host servers need so they can not only support their own VMs, but also accept the workload from a failing host. "If you're running all your servers at 80%, you won't be able to support that necessary redundancy," he says.
Most organizations will find that they need to dedicate at least a month to the capacity planning and testing phases to determine the appropriate physical-to-virtual server ratio for their environment, Ruest says.
4. Get Real-world Benchmarks From Peers
Finally, EMA's Mann advises IT teams to get involved in local user groups or attend large annual conferences, like VMware's VMworld or Citrix's Synergy, to meet peers with similar application environments. "Most attendees are more than willing to share information about their environment and experiences," he says. Rather than relying on vendor benchmarks, get real-world examples of what has worked and what hasn't at organizations with profiles similar to yours. "You'll have a better chance at setting realistic expectations."
Gittlen is a freelance technology writer in the Boston area. Contact her at firstname.lastname@example.org.