Servers are not human. They don't live and breathe. They just consume power and take up space. Do we really need so many? The virtualization revolution is about saving money, time, and floor space. Today's virtualization tools provide an efficient environment for testing, running, and managing applications – with lower electric bills and fewer hardware headaches. But is virtualization all good, or does it also open the door to new kinds of threats?

In this month's cover story, we examine the practical side of virtualization. We start with an introduction to some of the virtualization tools available for Linux. Then we take a closer look at a pair of popular open source virtualization alternatives: Xen and VirtualBox. Finally, we settle in for a look at the dark side of virtualization: the mysterious world of virtualizing rootkits.

Practical Virtualization

The virtualization paradigm has come down to Earth, leaving the lofty heights of Mount Olympus for real-world concerns like stability, performance, and ease of management. A virtualization system that wants to fulfill all of these requirements must be ready for:

• Provisioning and deployment – creating the required number of virtual instances as quickly as possible and with minimal need for manual configuration.
• Migration – converting physical machines to virtual machines – and vice versa – if necessary. The ideal is online conversion across the network, requiring as little personal intervention as possible. Equally important is ensuring freedom of movement for virtual machines during operations. Live migration is the basis of load balancing solutions with virtual machines.
• Administration – one of the most important considerations for most networks. It starts with capacity planning for virtual instances, through staging of storage and network resources, to countless settings for virtual hardware, physical hardware, users, and privileges.
• Workload management – handling multiple virtual machines or physical hosts and replacing instances in case of failure.
• Monitoring – providing detailed, realtime information on the status and resource usage of individual virtual machines.

The best solution for your network depends on your needs and your budget. Table 1 introduces some of the more popular virtualization options for Linux environments.

Management Tools

Many distributors have gone to great lengths to facilitate virtualization for their customers. Both the Novell and Red Hat enterprise distributions integrate a tool known as Virtual Machine Manager (or Virt-Manager). On SUSE, the tool is integrated with YaST. Virt-Manager gives users the ability to set up a Xen instance in a couple of simple steps. Besides Linux (Novell, Red Hat), Solaris 8-10, and Netware 6, the tool also works with various Windows versions – provided the CPU has the required virtualization support.

Red Hat Enterprise Linux 5.1 also uses Virt-Manager for managing virtual machines. Red Hat also offers a tool on top of Enterprise Server, called Advanced Platform, which builds clusters of virtual machines and is capable of migrating guests across the borders of a physical host.

Fedora 8 gives a clue to where things are headed for Red Hat. The latest Fedora includes the new Xen 3.1, and the Virt-Manager version supports both Xen and QEMU. Additionally, Red Hat is looking to improve the security of the administration tool, something that has been a mere sideshow in the past.

Google also has a tool for virtual cluster management, Ganeti, which is released under the GPL. Installing Ganeti is complicated because the tool requires half a dozen Python modules, which Novell, for example, does not provide in a single package.

Ganeti does not offer the convenience of a GUI. This said, the text-based Ganeti commands lend themselves to scripting solutions, which provides a means for integrating the tool with other open source utilities.

Another tool for managing virtual machines is openQRM, a powerful utility that manages images for virtual and physical machines on the same interface. Another contender is Qumranet's Solid ICE, which focuses entirely on desktop virtualization using the KVM kernel hypervisor.

At the other end of the scale are various small command-line tools, such as xen-tools, a collection of scripts used on Debian for creating and configuring virtual instances.

Hosting Farms

In many cases, users do not need to simulate a full-fledged computer with a custom kernel for every one of the virtual guests. Professional hosters, who are simply concerned with keeping their customers' web offerings apart, are quite happy with a single kernel that the guests can share.

Virtualization solutions such as OpenVZ, Virtuozzo, and VServer use this approach to the virtualization problem, giving users an amazing application density on normal hardware, with low overhead.

Many professional hosters use the commercial Virtuozzo product to give customers who want to manage their own systems access to virtual consoles. SWsoft, the company behind OpenVZ and Virtuozzo, first acquired a majority shareholding in Parallels, the desktop virtualization company, early in 2007 and is now using the Parallels brand name. The OpenVZ virtualization tools are available with any major distribution.