Server virtualization can be viewed as part of an overall virtualization trend in enterprise IT that includes storage virtualization, network virtualization and workload management. Server Virtualization technology enables multiple operating systems to run concurrently on a single physical system and share the underlying hardware resources, where each of the operating systems runs as a self content system (an isolated virtual environment). The virtual environments are also known alternatively as virtual private servers, partitions, guests, instances, containers or emulations. Server virtualization can be used for server consolidation with eliminating server sprawl, for rapidly provisioning the test / development servers, for deploying the business continuity solutions /disaster recovery solutions, and to centralize the server administration / management. For instance, you may have some dedicated / critical servers running in data centers using underlying hardware resources (i.e. CPU, memory, I/O etc.) up to 15-20% for most of the time except some occasions when it touches the upper limit. In such a scenario, server virtualization solution can facilitate server consolidation resulting in optimum utilization of resources reaching up to 70-80% thus making the remaining servers free for other IT purposes. Server virtualization approaches There are three popular approaches to server virtualization:

  1. Virtual Machine Model : Virtual machines are based on the host/guest paradigm. A virtual machine is a virtual execution environment (usually an application/operating-system), which does not physically exist but is created within another environment of execution. In this context, such virtual execution environment is termed as 'guest' and well existing environment as “host”. One 'host' can have multiple 'guests' (VMs) to run parallel and also allows pooling of physical resources ( CPU, memory, disk etc.) among them dynamically. This approach of server virtualization allows the host to run multiple guest operating systems (with even different flavors). It does not require any change in guest operating system. Guest works without having knowledge of host application/operating system. In this model, the two approaches that typically used are 'hosted' and 'hypervisor' architectures. A hosted approach provides partitioning services on top of a standard operating system and supports the broadest range of hardware configurations. In contrast, a hypervisor architecture is the first layer of software installed on a clean system (hence it is also referred to as a “bare metal” approach). When guest requires real computing resources from the host, it uses a hypervisor to coordinate. Hypervisor validates the guest-issued instruction sets while passing it to CPU for final execution. Hypervisor is also known as virtual machine monitor (VMM). Since it has direct access to the hardware resources, a hypervisor is more efficient than hosted architectures, enabling greater scalability, robustness and performance. Examples of virtual machine models are VMware and Microsoft Virtual Server.
  2. Para-virtual Machine Model : The Para-virtual machine (PVM) model is also based on the host/guest paradigm and uses a hypervisor/virtual machine monitor (VMM). Para-virtual machines are also capable of running multiple operating systems. In contrast to virtual machine model, in this model, guest operating system's code is modified (called porting) by VMM to achieve better performance closer to non-virtualized hardware. Examples of Para-virtual machines are Xen and UML.
  3. Operating system level Virtualization : Virtualization at the OS level varies the guest-host paradigm. In this architecture, host runs a single OS kernel as its core and exports operating system functionality to each of the guests. Operating system-level virtualization does not allow running different guest operating systems (i.e. different kernels), although different libraries, distributions etc. are possible. This single-kernel approach that eliminates system calls between layers, introduces only a negligible overhead and allows running hundreds of guests on a single physical server. OS-level virtualization systems have been designed to run multiple applications (with proper isolation and security) where different operating systems are not required but merely multiple instances of the same operating system are needed. Solaris Zones(Containers), Swsoft Virtuozzo, OpenVZ, Linux-VServer are examples of OS-level virtualization.

Choosing the Server virtualization approach: Server virtualization has various models; one may choose them as per their setup requirement. Virtual machine model with hypervisor may be used if guest Operating systems are of various types. This approach has wider scope of usage but poor performance. You may go for Para-virtualization model for better performance but it is supported only fewer Operating systems as it requires modification in guest Operating system. Operating System level virtualization may be opted for the scenario where you are using same operating systems for all the installations. This approach provides best performance & ease in resources administration as compared to other approaches of virtualizations. Salient Features of Server Virtualization: — Increases hardware utilization and reduces hardware requirements with server consolidation. (This is also known as Physical-to-Virtual or 'P2V' transformation.) — Reduces required data-center rack space, power, cooling, cabling, storage and network components by reducing the sheer number of physical machines. — Improves application availability and business continuity independent of hardware and operating systems. — Improves responsiveness to business needs with instant provisioning and dynamic optimization of application environments. As evident from a prediction by Gartner, Server Virtualization can be looked upon as a beneficiary solution for today's IT Enterprises, which helps substantially in reducing the total cost of ownership for their server infrastructure (specially in x86). Gartner Prediction By 2010, mainstream virtualization technology will embrace I/O virtualization, breaking the traditional bonds between physical servers, network switches and storage area network (SAN) switches; by 2010, shared operating system (OS) virtualization will become mainstream (for example, Solaris Containers and SWsoft Virtuozzo). Publication ID:G00143973 Dt:8/11/2006