Server virtualization

The term "virtualization" was coined in the 1960s, to refer to a virtual machine (sometimes called pseudo machine), a term which itself dates from the experimental IBM M44/44X system. The creation and management of virtual machines has been called platform virtualization, or server virtualization, more recently.

Platform virtualization is performed on a given hardware platform by host software (a control program), which creates a simulated computer environment, a virtual machine, for its guest software. The guest software, which is often itself a complete operating system, runs just as if it were installed on a stand-alone hardware platform. Typically, many such virtual machines are simulated on a single physical machine, their number limited by the host's hardware resources. Typically there is no requirement for a guest OS to be the same as the host one. The guest system often requires access to specific peripheral devices to function, so the simulation must support the guest's interfaces to those devices. Trivial examples of such devices are hard disk drive or network interface card.

Reasons for virtualization

In case of server consolidation, many small physical servers are replaced by one larger physical server, to increase the utilization of costly hardware resources such as CPU. Although hardware is consolidated, typically OSes are not. Instead, each OS running on a physical server becomes converted to a distinct OS running inside a virtual machine. The large server can "host" many such "guest" virtual machines. This is known as Physical-to-Virtual (P2V) transformation.

A virtual machine can be more easily controlled and inspected from outside than a physical one, and its configuration is more flexible. This is very useful in kernel development and for teaching operating system courses.

A new virtual machine can be provisioned as needed without the need for an up-front hardware purchase. Also, a virtual machine can easily be relocated from one physical machine to another as needed. For example, a salesperson going to a customer can copy a virtual machine with the demonstration software to his laptop, without the need to transport the physical computer. At the same time, an error inside a virtual machine does not harm the host system, so there is no risk of breaking down the OS on the laptop.

Because of the easy relocation, virtual machines can be used in disaster recovery scenarios.

Hardware-assisted virtualization

Hardware-assisted virtualization is a virtualization approach that enables efficient full virtualization using help from hardware capabilities, primarily from the host processors. Full virtualization is used to simulate a complete hardware environment, or virtual machine, in which an unmodified guest operating system (using the same instruction set as the host machine) executes in complete isolation. Hardware-assisted virtualization was first implemented on the IBM System/370, and was recently (2007) added to x86 processors (Intel VT or AMD-V).

Hardware-assisted virtualization is also known as accelerated virtualization; Xen calls it hardware virtual machine (HVM), Virtual Iron calls it native virtualization.

Hardware-assisted virtualization was first introduced on the IBM System/370 in 1972, for use with VM/370, the first virtual machine operating system. Virtualization was eclipsed in the late 1970s, with the advent of minicomputers that allowed for efficient timesharing, and later with the commoditization of microcomputers.

The proliferation of x86 servers rekindled interest in virtualization. The primary driver was the potential for server consolidation: virtualization allowed a single server to replace multiple underutilized dedicated servers.

However, the x86 architecture did not meet the Popek and Goldberg virtualization requirements to achieve "classical virtualization- :

- equivalence: a program running under the VMM should exhibit a behavior essentially identical to that demonstrated when running on an equivalent machine directly;

- resource control (also called safety): the VMM must be in complete