Introduction to Server Virtualization

What Is Virtualization? What Are the Benefits?

A typical data center contains many servers. Traditionally, since hardware was cheap and applications were complex, many administrators installed one application per server. Though this provided their applications with security and isolation from other processes, it resulted in server sprawl. Workload on these servers would vary widely depending on the time of day and user activity. Many of the servers would spend a large percentage of their time idling away, doing practically nothing. Average server utilization levels are typically about 10%. Server virtualization increases each physical machine’s utilization by dividing physical servers into multiple virtual servers. Each virtual server contains its own operating environment and applications; it looks and acts exactly like a real server.

There are three ways to run virtual servers: full virtualization, para-virtualization and OS-level virtualization. In all three, the physical server is called the host and the virtual servers are called guests.

Full virtualization uses a special kind of software called a hypervisor to supervise the guests. It interacts directly with the host’s CPU, disk and network access. The hypervisor keeps each of the guests independent of one another and unaware of the other servers running on the same machine. Full virtualization requires the most overhead, and thus allows the least number of virtual servers per physical server.

With para-virtualization, the guest servers are aware of each other’s existence. Though it also uses a hypervisor, para-virtualization doesn’t require as much overhead as full virtualization because the guests are each aware of the load that they’re placing on the host. This method requires that the host be running a specially modified version of the operating system to cooperate with the hypervisor, but it offers a considerable performance improvement over full virtualization.

An OS-level virtualization doesn’t have a hypervisor at all. The host’s operating system performs the hypervisor’s duties and interacts with the guests. The biggest drawback of OS-level virtualization is that all of the virtual servers must run the same operating system as their physical server, but the performance in the guests can be similar to an actual physical machine!

Which is best? It depends on the needs of the data center. If all the servers run the same operating system, the OS-level virtualization would be the fastest and allow the most virtual servers per host. If you need to combine a number of servers running different operating systems, both full virtualization and para-virtualization can do the job, but full virtualization is the older and more mature technology for doing so.

When to use Virtualization?

Virtualization is perfect for small to mid-size applications and services that don’t fully tax the hardware resources. Since virtualization is dividing a server’s resources into fractions, applications that are too resource-intensive can overwhelm the server. When the physical server can’t meet the demands of one of the virtual servers, all of the virtual servers will bog down.

Virtualization allows for server consolidation. If there are many applications that each only use a small amount of processing power, the administrator can consolidate several servers into one virtual machine. In a data center with hundreds or thousands of computers, the power and cooling savings would be significant.

In the past, when you upgraded your server hardware because it was obsolete, you were usually required to upgrade, or at the very least, extensively test all of the software running on it as well, because the old version of the software might not run the same on the new hardware. With virtualization, though, it is possible to maintain the same operating environment regardless of any changes to the physical infrastructure of the server. This has far-reaching implications for server upgrades, because upgrades are typically incredibly expensive, not just because of the cost of the actual hardware but because of the associated costs of testing to make sure that that everything behaves as it should.

Can Virtualization Make Your IT More Complex?

Virtual servers not only require ongoing maintenance and support but can also greatly increase the complexity of the IT environment. The combination of server virtualization and server consolidation can create a situation where specific physical machines become even more critical to the business. The way around this problem is to develop redundancy by running the same application on multiple virtual servers spread across different physical machines. If one server fails, or is taken offline for maintenance, the other computer would continue serving the application without any interruption in service.

In a large virtualized environment, patching and updating servers is more complex than in a traditional one. Not only do you need to upgrade the virtual machines but the physical server will also require updates. These will occasionally require that the host be restarted. This will also impact the virtual servers which will also need to be taken offline for the duration of the reboot. Having all of the servers hosted inside on machine down, even for just a restart, will impact the business a lot more than if it was just one server. Planned downtime, even for something as simple as a minor update, becomes something that needs to be carefully planned and orchestrated.

Backing up is also more complex. Not only does the physical machine need to be backed-up, but each of the virtual machines does too. Do you need to backup the entire virtual machine, or just the data created by the applications running on it? Backing up just the data will be faster, but it will more difficult to restore the machine to working state because you’ll also need to recreate the virtual server. Backing up the entire virtual machine, on the other hand, will require vastly more storage space to contain.

Monitoring is another issue that is more complex in this environment, because you’ll need an application to monitor both the physical and virtual machines. A server with ten virtual servers would require eleven instances of the monitoring application to make sure that everything is running normally. This will add some overhead that will need to be accounted for when planning the number of virtual services to distribute per server.

Finally, you can’t run everything in a virtual server; some applications are more suitable for virtualization than others. Server virtualization is the technology du jour, but it is definitely something to deploy after carefully thinking through the options and all of the ramifications.