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Machine Virtualization Techniques

There are two different techniques of server or machine virtualization the automated approach and the bare metal approach. The techniques differ depending on the type of hypervisor used. Although the techniques are different they have the same end or ultimate goal of creating a platform where multiple virtual machines can share the same system resources. Each technique is simply a different way of achieving this goal. ApproachIn this approach, an operating system is first installed on the physical machine to activate it. This OS installed over the host machine is referred to as the host operating system. The hypervisor is then installed over this host OS. This type of hypervisor is referred to as type 2 hypervisor or hosted hypervisor. Figure 7.4 represents the hosted machine virtualization technique. So, here the host OS works as the first layer of software over the physical resources. Hypervisor is the second layer of software and guest operating systems run as the third layer of software. Products like VMWare Workstation and MicrosoftVirtual PC are the most common examples of type 2 hypervisors. Benefits: In this approach, the host OS supplies the hardware drivers for the underlying physical resources. This eases the installation and configuration of the hypervisor. It makes the type of hypervisors compatible with a wide variety of hardware platforms.


                                      FIG 7.4:A model of hosted machine virtualization approach

Drawbacks: A hosted hypervisor does not have direct access to the hardware resources and hence, all of the requests from virtual machines must go through the host OS. This may degrade the performance of the virtual machines. Another drawback of hosted virtualization is the lack of support for real-time operating systems. Since the underlying host OS controls the scheduling of jobs it becomes unrealistic to run a real-time OS inside a VM using hosted virtualization. Type 2 or hosted hypervisors run within an operating-system environment Metal Approach: Removal of the Host OSIn this approach of machine virtualization, the hypervisor is directly installed over the physical machine. Since the hypervisor is the first layer over hardware resources, hence, the technique is referred to as a bare metal approach. Here, the VMM or the hypervisor communicates directly with system hardware. In this approach, the hypervisor acts as a low-level virtual machine monitor and is also called a type 1 hypervisor or native Hypervisor. VMware’sESX and ESXi Servers, and Microsoft’s Hyper-V, solutions are some examples of bare-metal hypervisors. Benefits: Since the bare metal hypervisor can directly access the hardware resources in most of the cases it provides better performance in comparison to the hosted hypervisor. For bigger applications like enterprise data centers, bare-metal virtualization is more suitable because usually it provides advanced features for resource and security management. Administrators get more control over the host environment. Drawbacks: As any hypervisor usually has a limited set of device drivers built into it, so bare-metal hypervisors have limited hardware support and cannot run on a wide variety of hardware platform


                               FIG 7.5:A model for the bare metal approach of machine virtualization

Type 1 or bare metal hypervisor does not use any host operating system

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