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The Many Choices in Virtualization

As has been discussed many times on this site, virtualization is one of the key enabling technologies for  green IT. The most common way in which virtualization produces its benefit is by providing a means for IT organizations to consolidate applications onto robust, power-efficient platforms, rather than letting them run on underutilized and inefficient legacy hardware. The power-consumption savings from this have been demonstrated repeatedly, and it is one of the fastest-ROI moves an IT site can make to begin realizing savings and migrating towards eco-responsibility.

Before plunging into virtualization, it's important to reconnoiter the landscape of choices and choose the software that fits your needs and budget. Unlike a few years ago, there are a multitude of choices today.

The software layer that makes virtualization possible is called the hypervisor. It's the hypervisor's job to run the various virtual machines (VMs) and make sure they have access to the underlying hardware resources. There are several kinds of hypervisors. Some such as Parallels and VMware Workstation are oriented to desktop rather than server deployments. They are not appropriate solutions for server consolidation. Desktop virtualization is for running other operating systems on a desktop machine. For example, Parallels enables Mac owners to run Windows applications. And VMware Workstation enables developers to test their software on multiple platforms on the very same workstation. Desktop hypervisors are not oriented towards running multiple VMs efficiently --- that is the task of a server-oriented hypervisor.

A server hypervisor runs multiple VMs and is frequently part of a package that includes tools to start and stop the VMs and perform a variety of related administrative tasks. The most established of the products in this category is VMware Virtual Infrastructure 3, frequently called VI3. The VI3 package includes the server layer, called VMware ESX Server, and the administrative tools. When VMware ESX server is installed, it takes over the entire hardware server. In other words, it requires you to dedicate the server to virtualization. It includes its own Linux kernel, so it literally runs directly on the bare metal. And to assure itself of this, it completely strips disks during installation. You can obtain a smaller version of just the VMware ESX server for free. It's called VMware ESXi and is available here at no cost. For many purposes, ESXi could well be entirely sufficient.

VMware Server (formerly named VMware GSX Server) is similar to ESX server, except that it installs on an existing operating system and does not strip the machine to run on the bare metal. It is available at no cost from here. Note that VMs built for VMware Server will not run on VI3; and vice-versa. This aspect is one that needs to be considered in any choice of hypervisor: You're locking yourself into a specific VM implementation. Note, though, that there are efforts underway to standardize VM formats so that they can be imported and exported, but until that reaches fruition, be aware of the VM restrictions on transportability.

Currently, VMware's largest competitor with regards to installed base is Xen, the open-source hypervisor that runs on Linux. The open-source version can be downloaded at no cost from In 2007, Citrix, a company that makes multiuser software products, bought Xen Source, which was the firm that sold a productized version of the open-source hypervisor. Citrix makes available a XenServer Enterprise Edition with paid support for $2600. A more modest Standard Edition costs less than $800. Both prices are below the levels what VMware charges, and so for sites that want paid support, Citrix might make a good alternative. Microsoft, which I discuss shortly, is also a contender, but the Citrix product has been field tested for several years, while Microsoft's product is in first release. For small deployments or to test out Citrix's hypervisor, you might consider the Express Edition, which is available at no charge.  This page shows the features of the entire range of Citrix hypervisors. One convenient feature of all the Citrix hypervisors is that they run the same VMs.

The final major option is Microsoft's Hyper-V, which is the company's new virtualization platform, replacing Virtual Server 2005. The hypervisor requires Windows Server 2008 running in 64-bit mode and currently officially supports only VMs running Windows XP, Vista, Windows Server, and SUSE Linux v. 10 and later. That's not a lot of options, especially on the Linux front. Microsoft will surely become an important player in virtualization, but as yet this offering is too young to assess and runs on a platform that is also too recent to recommend at the moment. I suspect that in a future column, I'll return to Hyper-V with more information from the field.

So, if you want to virtualize today, VMware and Xen are the proven offerings. I tend to give the nod to VMware because its free hypervisors are scalable and battle tested and have been in use longer than Xen's. But Xen's products should be an essential part of any pilot trial.

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