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Published on: 09/01/2008
Type of content: White Paper
Format: Adobe Acrobat (.pdf)
Length: 7
Price: FREE

Overview
The concepts behind application & operation system virtualization are not new concepts, they have been around long before server appliances & desktop PCs were readily available in our daily vocabulary. The recent rate of virtualization adoption however, especially that of software operating system virtualization, has grown exponentially in the past few years. According to Joe Tucci, the CEO at EMC®, most VMware® customers are planning on virtualizing 50% of their IT infrastructure within the next three years2. Virtual machines have finally come into their own, & are quickly moving into the enterprise data center & becoming a universal tool for all people & groups within IT departments everywhere.

There are two primary types of platform virtualization: transparent & host-aware (often referred to as paravirtualization). Transparent virtualization is implemented so that the guest is not aware that it's running in a virtualized state. The guest consumes resources as if it were natively running on the hardware platform, oblivious to the fact that it's being managed by an additional component, called the VMM (Virtual Machine Monitor), or hypervisor. The more standard forms of virtualization today, such as those by VMware, implement transparent hypervisor systems. These systems can be thought of as proxies: the hypervisor will transparently proxy all communication between the guest & the host hardware, hiding its existence from the guest so the guest believes it's the only system running on that hardware.

Host-aware implementations differ in that the guest has some form of virtualized knowledge built into the kernel; these can be considered "virtual self-aware" environments. There is some portion of the guest operating system kernel that knows about the existence of the hypervisor & communicates with it directly. Rather than transparent proxying of all communication, the guest OS will call the hypervisor directly, which will in turn manage the communication to the hardware. Xen (pronounced ‘zen'), a popular virtualization implementation for Linux, uses a hostaware architecture, requiring special hypervisor command code actively running in both the host & all running virtualized guests. Each form of virtualization comes with pros & cons, but both work equally as well. Transparent systems are the most portable for the guest, but sacrifice speed & are typically designed around much heavier hypervisors; host-aware systems are faster & more lightweight, but require guest modifications & can introduce security issues that transparent systems may not suffer from.

One of the driving factors in virtualization adoption is the open nature of hardware support for VMMs: Hardware platforms, which run & manage the primary host operating system, & the VMM are not specialized devices or appliances. Virtual host platforms can be any type of hardware that used today: single CPU desktop machines; laptops; x86 servers; SPARC servers; rack mounted appliances; etc. A normal user running Microsoft® Windows® XP Professional on their laptop can run multiple virtual instances of other operating systems--such as Linux, BSD, or Windows Vista -- using any number of freely available VMM software implementations. This flexibility, the move of virtualization software to everyday hardware, has allowed everyone direct & inexpensive access to run virtualized environments. While at first this access was relegated to technology professionals, such as Unix users who were required to run Windows as their base OS, it has quickly become the topic of IT managers. Platform virtualization provides an inexpensive mechanism to substantially expand server farms & data centers. Virtualization allows a company to purchase one high-end hardware device to run 20 virtual operating systems instead of purchasing 20 commoditized lower-end devices, one for each single operating platform.

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