Virtual Machine (VM) technology, the foundation for all virtual desktop solutions, allows a computer to simultaneously support and execute two or more computing environments, in which environment includes operating systems plus user applications and data. A VM is a virtual computing system that borrows computing resources (CPU, disk, memory, etc) from its host computer, co-existing alongside the host computer as a guest computer. Sharing computer resources can provide benefits to the end user, including:
Isolation The disparate environments operate without regard for the others. While some resources may be shared (for example, the keyboard, mouse, and screen), others are protected (for example, data files). In a VM environment, host resources need to be protected from concurrent access and control by both the host and the guest. In traditional VM systems, this isolation and control is provided by a special software program called a virtual machine monitor (VMM). The VMM may act as a peer of the host computing environment or may act autonomously from the host and guest computers.
Resource Balancing Each operating environment functions autonomously and therefore can have its resource consumption monitored and restricted, if desired.
Transportability/Mobility Certain VM configurations are considered mobile; that is, the entire environment can be moved from one host computer to a different one. Todays increasingly mobile world can benefit from such virtual machines, or VMs that can be transported from one host to another easily. Mobility is crucial when using VM technology to craft a virtual desktop solution. Although software products exist today that provide basic VM capabilities to current PCs, these products provide limited or no support for mobile VMs, due to their large software footprint. This is a key deficiency in these solutions.
There are many ways to implement a VMM and its supporting software. In this paper we will illustrate a variety of different VMM technologies, including:
Hypervisors VMMs that intercept and trap low-level CPU instructions, emulating any instructions that would violate isolation or cause system instability. Hypervisors can provide a complete virtualized desktop, with varying degrees of overhead / performance loss.
Paravirtualizers VMMs that intercept and trap low-level CPU instructions, failing any instructions that would violate isolation or cause system instability. Paravirtualizers can require that the guest desktop operating system be modified to avoid these privileged instructions. Paravirtualized systems can provide a complete virtualized desktop, with varying degrees of overhead / performance loss.
Workspace Virtualization Engines (WVE) VMMs that intercept and trap low-level OS API calls, emulating or redirecting those that would violate isolation or cause system instability. Some WVEs can provide the capability to virtualize privileged code modules and full operating system subsystems at a kernel level enabling the workspace to join an enterprise domain, have an isolated network stack and support applications such as endpoint security, databases, and PC management software that require drivers and security services. WVEs can provide a complete virtualized desktop, with little or no performance loss.
Application Containers Systems that intercept and trap the highest level OS API calls, emulating those that would violate isolation or cause system instability. Application containers cannot typically provide a complete virtualized desktop.
Emulators Systems that emulate entire hardware systems, including CPUs, I/O devices, etc. Emulators can provide a complete virtualized desktop, but at a tremendous performance loss.
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