#Virtual machine software

Written in 1974 for the third-generation computer systems of those days, they generalized the conditions that the software that provides the abstraction of a virtual machine, or VMM, must satisfy. Often referred to as the original reference source for VMM criteria, the Popek and Goldberg Virtualization Requirements define the conditions for a computer architecture to support virtualization. The VMMs discussed next manage the resource sharing among the VMs sharing a physical system. It is difficult to provide a service that checks file system integrity without the knowledge of on-disk structure.вЂќ As pointed out in, services provided by the virtual machine вЂњoperate below the abstractions provided by the guest operating system вЂ¦. Example: User-mode Linux.ДЄ semantic gap exists between the added services and the virtual machine. Performance as well as the challenges to support complete isolation of VMs make this solution less attractive for servers in a cloud computing environment. A price to pay for this simplicity is the increased overhead and associated performance penalty indeed, the I/O operations, page faults, and scheduling requests from a guest OS are not handled directly by the VMM. Another advantage of this solution is that the VMM could use several components of the host OS, such as the scheduler, the pager, and the I/O drivers, rather than providing its own.

The main advantage of this approach is that the VM is easier to build and install. The VMM shares the hardware with the existing OS. Examples: VMWare ESX, ESXi Servers, Xen, OS370, and Denali. A thin software layer that runs directly on the host machine hardware its main advantage is performance. Several organizations of the software stack are possible: Recall that a VMM allows several virtual machines to share a system. OpenVZ is licensed under the GPL version 2. These systems claim performance advantages over the systems based on a VMM such as Xen or VMware according to, there is only a 1 % to 3 % performance penalty for OpenVZ compared to a stand-alone Linux server. For example, OpenVZ requires both the host and the guest OS to be Linux distributions. Operating system-level virtualization allows a physical server to run multiple isolated operating system instances, subject to several constraints the instances are known as containers, virtual private servers (VPSs), or virtual environments (VEs). Systems such as Linux Vserver, OpenVZ (Open VirtualiZation), FreeBSD Jails, and Solaris Zones, based on Linux, FreeBSD, and Solaris, respectively, implement operating system-level virtualization technologies. A system virtual machine provides a complete system each VM can run its own OS, which in turn can run multiple applications.

Virtual machine