Hardware or OS virtualization software for servers? Server virtualization: the capability of partitioning a physical server into smaller virtual servers

Computer Technology Review, Feb, 2005 by Carla Safigan

Server virtualization has recently been gaining greater attention by enterprise IT professionals. There's a good chance that you have come across someone using, promoting or writing about the wares of consolidating servers or adding virtualization capabilities to their enterprise. What has been missing from the conversation is the fact that there are generally two different approaches to server virtualization: hardware emulation (VMware and Microsoft) and OS partitioning (such as Virtuozzo and VServer). Each technology has its advantages, and each was developed to address particular IT organizational needs.

Hardware Virtualization

Hardware emulation or virtualization is currently the best known virtualization technology. VMware is well known and Microsoft has just introduced a similar technology, its Virtual Server solution purchased from Connectix last year. Hardware emulation technology creates a full emulation of hardware at the host OS level, usually running as a single process inside the host OS. Full emulation typically runs some part of the guest OS code on native CPU, but some privileged instructions are emulated and handled separately. In addition to CPU, the software has to emulate BIOS, video adapters, network adapters, storage and input/output devices to provide a normal environment for the guest OS operation. The extent of the emulation allows almost any existing OS to run inside the virtual environment. Each virtual environment (generic term used across all technologies for virtual machine, virtual server, and virtual private server or VPS) has an entire OS structure, keeping all software and processes completely isolated. The primary use of this technology has been to provide multiple different OS flavors on the same physical piece of hardware. Multiple OSes are, in particular, a requirement in development and testing scenarios, where engineers are developing software simultaneously on different operating systems.

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OS Virtualization

OS partitioning or virtualization is a newer virtualization technology offered by Virtuozzo, Zones in Sun's Solaris OS, and also open source projects such as VServer. OS virtualization technology leverages a single common OS and creates separate, isolated virtual environments on a single physical server to share hardware, licenses and management effort. Processes and software are kept separate when necessary to distinguish between different virtual environments, but the design intent is to share the same software (when appropriate) across virtual environments. The primary use of this technology is to deploy production application and data on live servers. Each server runs an OS and may run other similar OSes, but since most enterprise applications run on one of a few OSes this tends to not be a barrier for production servers.

Which is the Best Approach for Production Servers?

Most IT organizations are considering virtualization as an answer to controlling runaway IT infrastructure growth. In many cases organizations are considering technology that could create drawbacks in other areas such as server performance or failing to fully capitalize on other benefits of virtualization such as decreased management cost.

Examine the full emulation partitioning technology as a solution for server consolidation, as an example, for existing and/or future applications. The host structure itself provides some initial overhead and typically 20%+ of the RAM is reserved for server overhead to run the virtualization software. Full hardware emulation is designed to mimic hardware to make it flexible enough to run any OS and completely separate virtual environments but making it inflexible from resource control and allocation perspective.

The OS partitioning implementation was designed to solve the issues of production servers so the architecture and design fit the consolidation criteria well. Typically when applications are deployed, they are grouped on the same OS for ease of management, maintenance and performance. The common OS affords a much more efficient server resource design and smaller overhead allowing hundreds of virtual environments (VEs) to run well on a single physical server and share resources effectively. In addition to this flexibility, resource allocations for VEs can be changed in real time without a reboot or service interruption. The low system overhead, dynamic resource management and high performance make VEs nearly as scalable as the host server in its native format.

Other Considerations?

Technology aside, each of these software solutions comes with a different set of features and capabilities. Here are a few additional considerations that may help in making the correct decision on a software virtualization solution:

* What OS do you require? Many of theses solutions have limited OS support, and few have the ability to manage virtual environments across OSes from a single management console.

* How many different OSes will you place on a single server? The first part of this article described the two major virtualization technologies; hardware emulation allows multiple OSes while OS virtualization places a single OS on a server.