How Many VMs Is Too Many?


Server consolidation is usually job one after a virtualization layer has been installed. Most vendors will spend a great deal of time explaining the management and flexibility benefits of their platform, but let's face it, cutting down the hardware budget is likely to remain the primary driver for some time.

But now that more and more enterprises are getting virtual experience under their belts, some are starting to question how much consolidation is enough.

While there is no rule of thumb as to how many virtual machines (VMs) can be housed on a typical server, it doesn't appear as if many of you feel you are pushing the envelope just yet. According to a recent survey from Forrester, about two-thirds of IT professionals say they want to increase the number of VMs per server, despite the fear that those VMs might not be adequately isolated from one another, causing application issues on one to affect its neighbors.

Cross-contamination of VMs might be the least of your worries if densities get too high, however. The primary concern is throughput. The virtual I/O industry has made quite a mark for itself with the ability to subdivide physical I/O resources into virtual ones, eliminating the bottleneck that occurs when too many VMs vie for a single HBA. And high-speed networking protocols like 10 GbE are helping virtualization platforms live up to their promises of 20 or more VMs per server.

The question, though, is whether that number is too high.

Greater efficiency and consolidation are fine under normal operating conditions, but as Burton Group analyst Chris Wolf points out to Virtualization Review, suppose the physical server goes down? Now, instead of two or three applications out of commission, you have 20. Not only are those users out of luck, but your VM availability is diminished as well.

There is also a misnomer about consolidation being a quick and easy process. As Vizioncore's George Pradel explains to Computerworld, shunting workloads from physical to virtual environments requires a lot more coordination than meets the eye. And more often than not, there is a substantial amount of downtime involved.

It's quite possible that, ultimately, managing virtual environments will be more of an art than a science. That's good for IT because it will place a high value on experience and judgment, rather than technical skills on one platform or another.

I'd be interested to hear from readers, though. How many VMs are you currently running per server, and how many do you think is appropriate?