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Book Excerpt

Comparing Virtualization Technologies

An overview of virtualization technology from a vendor-neutral perspective

Virtualization Magazine on Ulitzer

In this excerpt, we begin our exploration of several popular virtualization strategies and explain how each works. The aim is to bring you the operational information you need to make informed choices for your strategy. Each vendor's software has its own interface (console); its own methods of building, importing, and altering virtual machines (VM); and its own idiosyncrasies, tweaks, and tools.

This excerpt gives you a vendor-neutral but technical overview of the types of virtualization available. We approach the various types of virtualization from an application and performance perspective - in other words, a practical look at each technology and its implication for you. Each section also includes at least two representative examples of that technology.

Guest OS/Host OS
Virtualization aficionados perhaps know Guest OS/Host OS as classic or hosted virtualization. This type of virtualization relies on an existing operating system (the host operating system), a third-party virtualization software solution, and creation of various guest operating systems. Each guest runs on the host using shared resources donated to it by the host.

Guests usually consist of one or more virtual disk files and a VM definition file. VMs are centrally managed by a host application that sees and manages each VM as a separate application.

Guest systems are fully virtualized in this scenario and have no knowledge of their virtual status. Guests assume they are standalone systems with their own hardware. They are also not aware of other guests on the system unless it's via another guest's network services.

The greatest advantage of this kind of virtualization is that there are a limited number of devices and drivers to contend with. Each VM (guest) possesses a consistent set of hardware. The major disadvantage is that disk I/O suffers greatly in this particular technology. Nondisk operation speed, however, is near native. Therefore, we tell those who use hosted virtualization to interact with their VMs over the network using Windows Terminal Services (RDP) for Windows VMs or SSH for UNIX and Linux systems.

VMware Server
VMware Server is used throughout this book to illustrate virtualization techniques and technologies. It is a free offering from VMware and is considered an introductory package for use in small environments, testing, or for individuals. It has limited usefulness in large environments because of its memory limitations for VMs and sluggish disk performance. VMware Server supports 64-bit machines as hosts and guests.

Sun xVM (VirtualBox)
VirtualBox, which is now Sun xVM VirtualBox, is one of my favorite virtualization packages. Like VMware Server, it is free and cross-platform, but unlike VMware Server, it is open source. With adjustable video memory, remote device connectivity, RDP connectivity, and snappy performance, it may well be the best hosted virtualization package in your arsenal.

VirtualBox is best suited for small networks and individuals for the same reasons as VMware Server.

A hypervisor is a bare metal approach to virtualization. Bare metal refers to the server system hardware without any OS or other software installed on it. The best way to describe hypervisor technology is to draw a comparison between it and hosted virtualization. At first glance, the hypervisor seems similar to hosted virtualization, but it is significantly different.

A hypervisor is virtualization software that runs an operating system. Conversely, hosted virtualization utilizes an operating system and runs virtualization software as an application. The hypervisor software is installed to the bare metal, then the operating system is installed, which is itself a paravirtualized VM. The host operating system, if you can call it that, is designated as VM zero.

A new product, VMware ESXi, implements a bare metal hypervisor without a traditional operating system interface. It installs directly to the hardware in an almost impossibly small 32MB footprint. ESXi must be installed onto hardware that is virtualization optimized. VM management is performed via Direct Console User Interface (DCUI), which is the low-level configuration and management interface performed at the physical console of the server system. The VMkernel allows for remote management via a set of APIs and agents.

Citrix Xen
Xen versions 3.0 and earlier weren't particularly interesting to me because they were somewhat difficult to use and didn't seem to perform all that well for my specific applications. Xen 4.x products, however, have converted me heart and soul. The graphical interface is intuitive, fast, and extremely well thought out. The template engine in the new product is a pleasure to use, and provisioning a new VM with it is fast, fast, fast. If you have a need for high-end virtualization, you must check it out.

VMware ESX/VMware ESXi
Enterprise virtualization at its finest is brought to you by the people who breathed life into PC-based virtualization. ESX is a mature product that is rivaled only by Xen at this level of virtualization. Both products require 64-bit architecture, but ESXi has very special hardware requirements beyond those of ESX. ESXi is now a free product.

Microsoft Hyper-V
Microsoft steps up to the plate with its Windows 2008 Server family and Hyper-V virtualization solution where Citrix and VMware fall short: a Windows-based Enterprise virtualization product. Both Citrix Xen and VMware are Linux-based, which means that if you aren't familiar with Linux or UNIX commands, you may be better off using the Microsoft product.

This product, when more mature, promises to be a formidable challenge to VMware's and Xen's dominance in the Enterprise virtualization world.

Emulation refers to the capability to mimic a particular type of hardware for an operating system regardless of the underlying host operating system. For example, using an emulation virtualization solution, you can install a Sparc version of the Solaris operating system on a non-Sparc host computer. The emulation software runs as an application on the host system, but emulates an entire computer of another platform. The guest operating system has no awareness of its status as a guest operating system or that it is running in a foreign environment.

In some cases, hardware emulation can be painfully slow, but newer technology, updated emulation software and drivers, and faster 64-bit host processors make emulation a viable virtualization option - especially for those who need to develop drivers or technologies for other platforms without a large investment in support staff or hardware for them.

The best examples of hardware emulation software are Bochs and QEMU.

Bochs is a free, open source, Intel architecture x86 (32-bit) emulator that runs on UNIX and Linux, Windows, and Mac OS X, but only supports x86-based operating systems. Bochs is a very sophisticated piece of software and supports a wide range of hardware for emulating all x86 processors and x86_64 processor architecture. It also supports multiple processors but doesn't take full advantage of SMP at this time.

QEMU is another free, open source emulation program that runs on a limited number of host architectures (x86, x86_64, and PowerPC) but offers emulation for x86, x86_64, ARM, Sparc, PowerPC, MIPS, and m68k guest operating systems.

Microsoft Virtual PC and Virtual Server
Virtual PC is a free virtualization software package from Microsoft. Virtual PC uses emulation to provide its VM environment. These are good solutions for hosting a few VMs on a Windows XP Workstation or Windows 2003 Server. It isn't a large environment solution by any stretch of the imagination, but it can get some VMs up and running cheaply and in very short order.

VM performance on these products is surprisingly good for Windows VMs. It is difficult, if not impossible, to tell that you are using a VM when connecting over the network. Console performance tends to be a little sluggish at times - so whenever possible, minimize the console and use RDP to connect to your virtualized Windows systems.

Kernel-level virtualization is kind of an oddball in the virtualization world in that each VM uses its own unique kernel to boot the guest VM (called a root file system) regardless of the host's running kernel.

Linux KVM (Kernel Virtual Machine) is a modified QEMU, but unlike QEMU, KVM uses virtualization processor extensions (Intel-VT and AMD-V). KVM supports a large number of x86 and x86_64 architecture guest operating systems, including Windows, Linux, and FreeBSD. It uses the Linux kernel as a hypervisor and runs as a kernel loadable module.

User-Mode Linux
User-mode Linux (UML) uses an executable kernel and a root file system to create a VM. To create a VM, you need a user-space executable kernel (guest kernel) and a UML-created root file system. These two components together make up a UML VM. The command-line terminal session you use to connect to the remote host system becomes your VM console. UML is included with all 2.6.x kernels.

More Stories By Kenneth Hess

Kenneth Hess, a consulting engineer at a global IT outsourcing firm, has been involved in virtualization and running VMs since becoming an official beta tester for VMware's first product. He uses Microsoft's Virtual PC on a daily basis to maintain connectivity to corporate networks and test new software. He is also currently involved in an enterprise virtualization project utilizing VMware Server and Xen Server 4.x to migrate and consolidate aging hardware onto a virtual platform. Hess has many years of experience in IT areas ranging from management to development to technical writing.

More Stories By Amy Newman

Amy Newman is the managing editor of ServerWatch and Enterprise IT Planet. She has been covering virtualization since 2001, and is the coauthor of Practical Virtualization Solutions, published by Pearson in October 2009.

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Most Recent Comments
pankakekid 12/31/09 10:07:00 AM EST

Good article, with one major caveat.

KVM is a game-changer and at least deserves as much billing as Xen. Xen is backed by Citrix and has more market share, KVM is backed by an equally significant corporate entity: Red Hat and has a far superior architecture.

KVM's inclusion in the mainline Linux kernel means it will be the open source hypervisor of choice for Linux distros in the future, as it already is for Red Hat Enterprise, Fedora, and Ubuntu.