Hypervisor comparisons: It's harder than you think

In the technology game, everyone has his whipping boy and his favorite son. Get the facts and make an informed but difficult decision.
Written by Ken Hess, Contributor

[UPDATE] Hyper-V 3.0 updated support information. See updates below the Hypervisor Comparison Table.

If you're having trouble deciding on a virtualization technology for your business, you're not alone. It isn't as easy as some would have you believe. Prejudices and preferences aside, the major hypervisor technologies tend to run together when you look at some of the basic features. And, the equation becomes even more complicated when you factor in licensing costs, virtual machine density, training costs, hardware requirements, performance and, if you'll pardon the pun, a host of other factors.

Look at the "Hypervisor Comparison Table" below to see a few factors in a comparison matrix that I've devised for you. You can see that there's little difference in technical specifications among the five major technologies shown. There are no "runaway" winners when comparing on this scale.

Cost Comparisons

If you're basing your selection purely on the basis of price, then don't listen to marketing hype, opinion or rhetoric. You need to contact the companies whose technologies you're interesting in checking out and have them give you pricing. If you haven't done so already, take a complete inventory of your physical systems to determine which ones will find roles as hosts, which ones will be decommissioned, which ones can be consolidated and which ones must remain as physical systems.

A physical hardware inventory includes system make, model, CPUs, Cores, memory, disk and network.

Take note of the guest operating system for those servers as well so that you can make an accurate licensing assessment.

Finally, present your results to your selected vendors and wait for the numbers to come in.

Hypervisor Comparison Table

Feature XenServer 6.0 Hyper-V 2.0 Hyper-V 3.0 vSphere 5.0 RHEV 3.0
Cores/Host 160 64 160 160 160
RAM/Host 1TB 1TB 2TB 2TB 2TB
CPUs/VM 16 4 32 32 64
RAM/VM 128GB 512GB 512GB** 1TB 64GB
VM Disk 2TB 2TB 2TB** 2TB 2TB*
VM Live Migration Yes Yes Yes Yes Yes
**Hyper-V 3.0 Update: VMs now support 1TB RAM and all OSs support 64TB disks.

Virtual Machine Density

Pay close attention to virtual machine density numbers that the vendors provide you. Depending on the virtual workloads you intend to deploy and your host hardware, your virtual machine density should fall into the general range of 10 to 40 server VMs per host. If you're considering a technology switch, you need to pay very close attention to virtual machine density numbers as that single factor can significantly drive up your hardware and host licensing costs.

Virtual Machine Licensing Costs

Host technology can alter your costs as well. Microsoft's Data Center Edition of Windows Server carries an unlimited number of Windows guests. So, depending on how your host hardware is configured, you can 'sometimes' save money with a Hyper-V solution. If your host technology doesn't offer unlimited Windows guests, then you need to carefully calculate your required licensing fees. Of course, if you convert licensed physical systems to virtual machines, then your licenses follow the system. You don't have to purchase a new license.

Hidden Costs

There are some hidden, or at least some unexpected, costs associated with virtualization or switching from one technology to another. One of those costs is training. If you've invested in VMware training for your technicians and system administrators, you might find that the extra expense to train isn't worth the switch. Alternatively, virtualization technologies have many similarities and trained professionals in one technology can fast track themselves onto another.

Another often unexpected cost is hardware upgrade or hardware replacement. Virtual machine hosts must be newer technology, meaning that they must have multiple 64-bit, virtualization-capable CPUs, multiple cores, a lot of RAM and, for storage, you'll need a fast storage area network (SAN) or network attached storage (NAS). Having networked storage also might mean that you need to hire storage expertise or pay for SAN technology training.

If you also decide to switch from standard server technology (1u, 2u, 4u form factor hardware) to blade servers, you'll have to factor in specialized power, network and rack configurations for your systems. Blade servers are a very good alternative (added expenses notwithstanding) to standard architecture because of the server density (10 to 14 per enclosure), ease of setup and rapid provisioning.


Basically, your choices fall into two categories: Status quo or Switcheroo. If you maintain status quo, you know what your costs are, you know what to expect from your vendor and you have experience with the product and its flaws and features. Going with the old switcheroo might leave you long on promises but short on funds. If you'll notice, marketing literature never states, "Hey, we're not number one but we're still a good alternative." Everyone's technology is better, cheaper and just short of being the answer to the Ultimate Question. If you have yet to select a hypervisor technology, remember that you're buying into a genre of sorts. You're buying into a whole host (again, pardon the pun) of product formulations and mindsets.

There's no single correct answer for every need. In fact, some companies elect to use multiple virtualization technologies. For example, it might be less expensive for you to run all of your new Windows guests on Hyper-V and all of your previously licensed guests on Red Hat's KVM. You'll have to do your own analysis.

Don't allow any one vendor to tell you that just because their solution is cheapest/best/least irritating/most popular/lowest maintenance/etc. My best advice is to put pencil to paper, assess the features, apply a price per feature for the technologies you down-select and make your decision based on hard facts not FUD, conjecture, fanboydom or Twitter frenzy. Trust your eyes not your ears but It's harder than you think.

* No direct documentation could be found for KVM or QEMU max disk size.

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