Virtualization: Tips for avoiding server overload

Virtualization: Tips for avoiding server overload

Double-digit physical-to-virtual server ratios are things of the past

Continuous monitoring is key

At Network Data Center Host, a Web service provider in San Clemente, Calif., the IT team quickly learned that when it comes to virtualizing mission-critical applications, you have to consider more than just RAM. "We originally thought, based on available RAM, we could have 40 small customers share a physical server. But we found that with heavier-used applications, it's not the RAM, it's the I/O," says CTO Shaun Retain.

The 40:1 ratio had to be pulled back to 20:1 at the greatest, he says. To help with that effort, the team has written a control panel that allows their customers to log in and see how their virtual machine is handling reads, writes, disk space usage and other performance-affecting activity. In addition, NDC Host uses homegrown monitoring tools to ensure that ratios aren't blown by a spike in a single VM's traffic.

Pund-IT's 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 co-exist 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 the Resolutions Enterprise 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.

More virtualization-management tips

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 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 they need to dedicate at least a month to the capacity planning and testing phases to determine the appropriate P-to-V server ratios for their environment, Ruest says.

Finally, EMA's Mann advises IT teams to seek out peers with similar application environments at large annual meetings like VMware's VMworld conference or Citrix's Synergy, or through local user groups. "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 whathas workedand what hasn't at organizations with your same profile. "You'll have a better chance at setting realistic expectations."

Gittlen is a freelance technology writer in the greater Boston area who can be reached at

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