Don't fall into the trap of overestimating physical-to-virtual server ratios; it can come back to bite you, big time.

Computerworld - As virtualization stretches deeper into the enterprise to include mission-critical and resource-intensive applications, IT executives are learning that double-digit physical-to-virtual server ratios are things of the past.

Virtualization vendors may still be touting the potential of putting 20, 50, or even 100 VMs (virtual machines) on a single physical machine, but IT managers and industry experts say those ratios are dangerous in production environments, causing performance problems or, worse, outages.

"In test and development environments, companies could put upwards of 50 virtual machines on a single physical host. But when it comes to mission-critical and resource-intensive applications, that number tends to plummet to less than 15," says Andi Mann, vice president of research at Enterprise Management Associates (EMA) in Boulder, Colo.

In fact, EMA conducted a study in January 2009 of 153 organizations with more than 500 end users and found that on average they were achieving 6:1 consolidation rates for applications such as ERP, CRM, e-mail and database.

The variance between the reality and the expectations, whether it's due to vendor hype or internal ROI issues, could spell trouble for IT teams. That's because the consolidation rate affects just about every aspect of a virtualization project -- budget, capacity and executive buy-in. "If you go into these virtualization projects with a false expectation, you're going to get in trouble," Mann says.

   

Indeed, overestimating P-to-V ratios can result in the need for more server hardware, power consumption, heating and cooling, and rack space -- all of which cost money. Worse yet, users could be impacted by poorly performing applications. "If a company thinks they're only going to need 10 servers at the end of a virtualization project and they actually need 15, it could have a significant impact on the overall cost of the consolidation and put them in the hole financially. Not a good thing, especially in this economy," says Charles King, president and principal analyst at consultancy Pund-IT in Hayward, Calif.

Key apps will fight for server space

So, why the disconnect between virtualization expectations and reality? King says up to this point, many companies have focused on virtualizing low-end, low-use, low I/O applications such as test, development, log, file and print servers. "When it comes to edge-of-network, non-mission-critical applications that don't require high availability, you can stack dozens on a single machine," he says.

Bob Gill, managing director of server research at consultancy TheInfoPro, agrees. "Early on, people were virtualizing systems that had a less than 5% utilization rate. These were also the applications that, if they went down for an hour, no one got upset," he says.

source http://www.computerworld.com/s/article/9138495/Virtualization_Tips_for_avoiding_server_overload