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How "Free" are the free server virtualization products?

Some years ago the big three competitors in the Server Virtualization market Citrix, VMware and Microsoft decided to give away (a part of) the product for free. In this article I'm going to describe what the free product actually offers and how useful these free versions are. I will just mention and compare the available features. I'm not going to discuss how these features are implemented and how good they function/perform. When considering a free hypervisor you should always test the product to see if the offered features offer the functionality you would like to have.

Citrix XenServer


Let's start with Citrix XenServer. Citrix offers a good overview of the features of the different version in a feature matrix. Just like all other suppliers lot of difficult words are being used to describe the features. The free edition of example offers ResilentDistrubuted Management Architecture, which actually means that the management functionality is taken over by another host (if more hosts are available). This is also based on the architecture how XenServer works.

Translated to normal words, this means that a XenServer environment based on the free edition can be managed from a central point. To accomplish this also the management tool XenCenter is available to manage the Free edition hosts. In my opinion this is a big advantage, because central management is key in infrastructures.  With the free edition of XenServer you also have the possibility to use the conversion tools. With the conversion tools you can P2V (convert a physical machine into a virtual machine) server into a virtual machine on the XenServer infrastructure.

One of the big advantages of server hypervisors is the possibility to take snapshot of a virtual machine and revert back to the state of the snapshot if necessary. This is also available in the free edition of XenServer. Together with these techniques and the XenCenter Management console the product also offers rolling out virtual machines using a template (which can be optimized for XenApp virtual machines).

Another big advantage of hypervisors is the possibility to move a virtual machine from one host to another. In XenServer this is called XenMotion Live Migration and is indeed available in the free edition.

The last option offered in the free edition is management integration with Microsoft System Center VMM, but this product is not free available so this does not count in building a free server hypervisor infrastructure.

Microsoft Hyper-V

Microsoft was actually the one that started the free hypervisor market by announcing Hyper-V which was freely available within Windows Server (and nowadays also as a stand-alone server). I will use this stand-alone server for this article to keep the completion as clear as possible.


Currently Hyper-V in Windows 8 is hot and offers lots of improvements, but during the writing this is still a beta so I will use the facts from the current available Hyper-V (version 2).


The first thing that is noticeable is that Microsoft is offering High availability in the free edition. In other words you can cluster the Hyper-V hosts and if one fails the other hosts will start over the virtual machines. You (now) need to use Microsoft FailOver Clustering, but that's bundles in the Hyper-V package, so yes Microsoft offers High Availability for free.

Secondly Microsoft also offers Live Migration in the free edition, so you can move a VM while running from one Hyper-V host to another Hyper-V host. Also the feature Dynamic Memory introduced is available in R2. With this feature you can assign a startup memory amount and specify how much additional memory the machine is allowed to use, but this part is not reserved directly. Again this is not offered by the other suppliers in the free edition. Hyper-V also logically offers Virtual Machine Snapshots techniques.

Because Hyper-V is logically based on Microsoft Windows you have additional features that are by default available in the OS like Bitlocker support and Live Backup support (using Volume Shadow Service).

Actually there is not really a paid version of Hyper-V. Where Hyper-V is embedded in the OS version (where you pay for) you receive (some) licenses for running Windows server based virtual machines, but not any additional functionality is offered. Microsoft is taking another approach to the hypervisor model than both Citrix as VMware.

The only thing Hyper-V is missing is full central management. This is offered via Virtual Machine Manager, which need to be purchased. Actually this product is not really expensive, but not free where it's all about in this article.

VMware vSphere Hypervisor

Last but not least is the pioneer in the hypervisor market. VMware was the first company introducing server virtualization and is still their main business. That's actually the first big difference between VMware and their main competitors. Both for Microsoft as Citrix is the hypervisor not their core business and is mainly an add-on to fulfill their portfolio. VMware offers in the commercial editions definitely the most features and products around the hypervisor platform. But what brings the free edition.

If you take a look at the website of VMware it not that easy to find much information about the free edition for example which features are offered in the free edition. Comparison matrixes only show the paid versions; personally I don't like such approach.

The free vShpere Hypervisor (formerly called free ESXi or ESXi Single Server) is the only free hypervisor (as far as I can found in the specifications) that has a limit on the hardware. The host memory is limited to 32GB, also you are not allowed (but you can do that) to assign more than 32 GB in total to the virtual machines.

The free edition does not offer central management; you can connect with the vSphere Client to every single host. For central management you need to purchase vSphere Server.

Except Snapshots the product does not offer additional functionality like vMotion, High Availability or other features you could know from the commercial editions. Also you should not forget that also the vCLI is limited to read-only, while the vCLI is used by several commercial back-up solutions (so these cannot be used on the free edition as well).

In my opinion vSphere Hypervisor is just a response to Microsoft offering a free hypervisor to the try the hypervisor for a first glance. Just like Citrix you could buy a license for the product and you can use the new features without reinstallation or reconfiguration steps. 


There are big differences in the features offered between the free hypervisor editions by VMware, Citrix and Microsoft. The VMware vSphere Hypervisor offers a very limited feature set and is mostly only usable for evaluating purposes or running test environments. On the other hand Microsoft is offering the most features in their hypervisor. Actually the options offered are suitable for many infrastructures and can be used to deploy a production environment. The only thing that is really missing in Hyper-V is a full central management in the free edition. Citrix is somewhere in the middle with offering functionalities, but offers central management for the free edition. In my opinion central management is key for a hypervisor infrastructure.

As mentioned in the beginning of the article I'm not going to discuss the quality, performance, usability, ease of operation and so on of the hypervisors, and just compare them on the functionalities offered in the free edition. With that on mind Microsoft will be the winner purely on the functionalities and if central management is an import feature Citrix is the way to go. VMware is far behind and personally I think the vSphere Hypervisor is just a loss leader for purchasing the commercial editions.

Always keep in mind when you start with a free hypervisor that overtime you may need to have additional features. VMware offers the most features in their product while Microsoft does not offer any additional features in their product. Citrix again is in the middle with the feature set. But just should also think about the necessity of features as I described earlier in this article series The need for advanced features within server virtualization.