Advantages and disadvantages of server virtualization

Virtualization is a hot topic, and server virtualization is a top priority for IT budgets in 2008. Although virtualization started out as a technology used mostly in testing and development environments, in recent years it has moved toward the mainstream in production servers. In this installment of our Virtualization Projects Hot Spot Tutorial, we will review the advantages and disadvantages of server virtualization for you and your clients.

Virtualization is fairly well-established, but there is still room for the market to grow. About 75% of large enterprises use virtualization to some extent, but most of those have virtualized only about 10% of their servers, said Barb Goldworm, president of Focus Consulting, a research firm based in Boulder, Colo. Small and medium-sized businesses (SMBs) are also getting into virtualization; when they do, those companies tend to virtualize much more of their infrastructure, since it is simpler and easier to migrate to a virtual environment, she said.

Even if your clients don't ask about virtualization, going over the advantages and disadvantages of server virtualization could persuade them to consider it, said Marvin Huffaker, president of Marvin Huffaker Consulting Inc., an IT consultancy in Chandler, Ariz. Huffaker said clients usually come to him asking about a server they need to upgrade or replace, and he is the one to suggest migrating to a virtualized server. Once clients see that virtualization works reliably, they usually spring for it, he said.

The biggest advantage of virtualization is the ability to consolidate several physica...

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But consolidation presents both the advantages and disadvantages of server virtualization. Besides using resources more efficiently, virtualization can also be risky because it puts more of your client's eggs in one basket. If the host machine breaks or needs to be taken offline, several virtual servers will go down. You can solve this by setting up a redundant server; if the primary server goes down, the secondary server will run the VMs until the primary one is fixed.

A robust virtualization project should include at least two physical servers that both have access to the same storage device, said Scott Gordon, sales engineer at ActivSupport, a San Bruno, Calif., networking consultancy. Because VMs are just data files, a shared storage system lets both physical servers, or hosts, access the same VMs, making it much quicker to recover if one server fails. A storage area network (SAN) is best for this, since it allows for nearly instantaneous recovery, but a network-attached storage (NAS) device or even backups can bring your disaster recovery plan's recovery time objective (RTO) down to minutes instead of days, Gordon said.

In fact, disaster recovery is one of the common reasons clients cite for adopting server virtualization, said Ty Schwab, founder and senior consultant of Blackhawk Technology Consulting LLC, a Eugene, Ore., IT consultancy. He worked with the Alaska Railroad to set up a virtualized server with an alternate site at a building across the tracks. The system could fail over within an hour, he said.

Performance is an important factor in weighing the advantages and disadvantages of server virtualization, especially for applications like databases that require a lot of disk activity. The prevailing wisdom is that databases should still run on dedicated physical servers, and time-sensitive applications like Voice over IP (VoIP) may also be poor candidates for virtualization, Gordon said. For those servers you do virtualize, virtualization software lets you manage load balancing by allocating resources to each VM as needed.

The two major virtualization applications are VMware's ESX Server and Citrix's XenServer. VMware's early stake in virtualization has given it market leadership, but its software is proprietary and relatively expensive. XenServer is based on the Xen open source project, which was owned by XenSource until that company was bought out by Citrix in 2007. VMware ESX Server and XenServer Enterprise Edition both support live migration of VM images, meaning that IT managers can transfer a VM from one host to another without any downtime.

VMware ESX Server is generally seen as the safer choice, and XenServer's management tools are not as advanced. The two applications differ in their technologies, too: VMware uses a "bare metal" hypervisor -- the virtualizing layer -- which makes an operating system (OS) think it's running on a physical server, whereas Xen uses paravirtualization, which can be faster but requires the guest OS to support being virtualized; older Windows systems don't support this.

As with many other open source projects, the Xen project is also the foundation for other vendors, such as Virtual Iron, whose software is more expensive than XenServer but less than VMware. VirtualIron has better management tools than XenServer, which Huffaker said does not handle more than three or four VMs well.

Microsoft also has a virtualization engine called Virtual Server, which it released in 2005, and a hypervisor known as Hyper-V (formerly code-named "Viridian") that is built into Windows Server 2008 or available as a standalone product. Although Microsoft is relatively new to virtualization and lags behind the other vendors' products, the company is known for putting out a comparatively weak first version of a product and building it into a serious competitor, Gordon said.

Server virtualization is only one part of the greater virtualization puzzle on the systems side. As your client considers the advantages and disadvantages of server virtualization, you should also keep desktop and application virtualization in mind. Desktop virtualization is similar to server virtualization, in that the operating system each user works with exists as a VM, but there are distinct differences in its benefits and implementations. Application virtualization is different in that only one application, instead of a whole OS, exists within the virtualized environment. In the next installment of this Hot Spot Tutorial, we will look at some of the different ways of implementing virtual desktops and each one's advantages and disadvantages.