VMTN Blog

[Updated below]

Eric Horschman over at our new VMware: Virtual Reality blog today writes about one way to evaluate the true cost of your hypervisor -- and it's not the sticker price. Leaving aside both functionality and operational costs, take a look at cost per virtual machine. Link: Cheap Hypervisors: A Fine Idea -- If You Can Afford Them

VMware also has a great TCO/ROI calculator to help you decide, but if all that seems like too much work, let me propose a simpler metric for comparing hypervisors – cost per virtual machine.

Cost per VM is not that hard to measure: just add up the costs for the server, the virtualization software, the operating systems and the application software; then start adding VMs running your workloads until they can no longer meet your required service levels. We’ve actually done that work for you and you might find the results surprising.

We took a common dual socket server with 4GB of RAM and tried the test with ESX Server 3, Citrix XenServer v4 and Microsoft Hyper-V beta. We created and powered on 512MB Windows XP VMs running a light workload and kept adding them until the server couldn’t take any more. Our Hyper-V and XenServer tests topped out at six and seven VMs respectively, which was expected. You see, both those products subtract the full amount of memory allocated to each running VM from the host’s physical RAM. When you factor in the additional memory required by the hypervisor and the management OS, there’s room left for at most seven VMs. In fact, XenServer and Hyper-V will flat out refuse to let you power on an additional VM with a warning that memory resources have been exhausted, as shown in the screen shots below. XenServer and Hyper-V can’t do what we call “overcommiting” memory and that should strike you as tremendously wasteful when most data center VMs are lightly utilized.

Eric goes on to explain VMware's transparent page sharing and balloon memory driver that together allow ESX Server to get roughly double the number of VMs per server than other hypervisors that don't have this technology.

Finally, we do the division and find that even our high-end VI3 Enterprise bundle beats a free hypervisor in cost per VM! Going with any other hypervisor means you’ll need more hardware, network and storage connections, switch ports, floor space, power and cooling to support a given population of VMs. That should make your decision easy if all you’re doing is simple server consolidation, but there’s more to consider. VI3 Enterprise includes a powerful array of virtual infrastructure services like VMotion, DRS, HA and more that let you automate, optimize and protect your operations, and those features put us far ahead of the offerings from the Xen vendors and Microsoft.

Read the whole thing.

Update: Eric seems to have touched a nerve, so he's going into more detail. Make sure you check out the comments on both posts -- interesting discussion and real customers. Link: More on VMware Memory Overcommit, for Those Who Don't Trust the Numbers | VMware: Virtual Reality.

More on VMware Memory Overcommit, for Those Who Don't Trust the Numbers

James O’Neill of Microsoft had a pretty sharp reaction to my post. He accused me of cooking the numbers to exaggerate the cost saving benefits of VMware’s memory overcommit feature. To help James understand just how memory overcommit works, I’ve taken the numbers out of the argument and have used simple algebra to compare the TCO per VM for a host with either VI3 or Microsoft hypervisors. Let’s do the math…