OPERATING SYSTEM LEVEL VIRTUALIZATION: REMOVAL OF THE HYPERVISOR
Operating system-level virtualization (also called system-level virtualization) works in a totally different way than the virtualization techniques discussed. Here, the host–guest paradigm does not work. The virtual machine terminology is also not used here. In this kind of virtualization technique, no hypervisor is used and the virtual servers are enabled by the kernel of the operating system of the physical machine. The approach is shown in Figure 7.8
FIG 7.8.A model of operating system-level virtualization approach
Here, the kernel of the operating system installed over the physical system is shared among all of the virtual servers running over it. Since all of the virtual servers share a single kernel, it is evident that all of them will have the same OS as the parent system. The goal of this approach is to create multiple logically distinct user-space instances (virtual servers) over a single instance of an OS kernel. This approach is also known as operating System Virtualization or Shared kernel approach. Virtualization solutions such as FreeBSD’s jail, LinuxVServer, and OpenVZ are a few examples of OS-level virtualization. All of them can run logically distinct user spaces on top of a single kernel. Advantages: The advantage of OS-level virtualization is that it is lighter in weight since all of the virtual servers share a single instance of an OS kernel. This enables a single physical system to support many other virtual servers than the number of complete virtual machines it could support. Limitations: All virtual machines have to use the same operating system (due to the sharing of kernel). Although different distributions (like Linux distribution) of the same system kernel are allowed. OS-level virtualization facilitates the creation of multiple logically distinct user-space instances rather than creating complete VMs.
Operating system-level virtualization (also called system-level virtualization) works in a totally different way than the virtualization techniques discussed. Here, the host–guest paradigm does not work. The virtual machine terminology is also not used here. In this kind of virtualization technique, no hypervisor is used and the virtual servers are enabled by the kernel of the operating system of the physical machine. The approach is shown in Figure 7.8
FIG 7.8.A model of operating system-level virtualization approach
Here, the kernel of the operating system installed over the physical system is shared among all of the virtual servers running over it. Since all of the virtual servers share a single kernel, it is evident that all of them will have the same OS as the parent system. The goal of this approach is to create multiple logically distinct user-space instances (virtual servers) over a single instance of an OS kernel. This approach is also known as operating System Virtualization or Shared kernel approach. Virtualization solutions such as FreeBSD’s jail, LinuxVServer, and OpenVZ are a few examples of OS-level virtualization. All of them can run logically distinct user spaces on top of a single kernel. Advantages: The advantage of OS-level virtualization is that it is lighter in weight since all of the virtual servers share a single instance of an OS kernel. This enables a single physical system to support many other virtual servers than the number of complete virtual machines it could support. Limitations: All virtual machines have to use the same operating system (due to the sharing of kernel). Although different distributions (like Linux distribution) of the same system kernel are allowed. OS-level virtualization facilitates the creation of multiple logically distinct user-space instances rather than creating complete VMs.