91d437f0fb56cfb5b64de81b60bd6738

Source: http://searchservervirtualization.techtarget.com/…

Author: Bernard Golden

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Even though VMware and Xen both provide an enterprise-focused server virtualization product, Xen’s approach to implementing virtualization has made for some challenges. VMware bases its product on hardware emulation, in which VMware provides a software layer that “looks” like an x86-based machine to a guest operating system. VMware cleverly patches the running guest operating system so that it interacts with the hypervisor, which in turn mediates between the guest operating system and the underlying hardware. This is a powerful technique that allows unmodified operating systems to run as guest machines; however, it takes a toll on performance due to the hardware emulation the hypervisor provides.

Xen’s product, by contrast, operates more like a traffic cop, multiplexing access to the underlying hardware resources. Xen dubs this approach “paravirtualization,” and one of the primary benefits is that the hypervisor is a very skinny piece of code which imposes little overhead. Tests run against paravirtualized guest operating systems indicate a trivial amount of virtualization performance hit on the order of less than 5%.

A drawback to Xen’s approach is that the thin hypervisor requires modification of the guest operating systems so that they run as paravirtualized guests. Specifically, this requires patching the kernels of the guest operating systems to allow interaction with the control structures of the Xen hypervisor. Another drawback to Xen’s thin architecture is that underlying services must be provided by a privileged guest operating system. (In Xen parlance, a privileged operating system is called a Dom0, and a regular guest operating system is called a DomU.) The privileged guest requires a patched kernel as well, since it must access the same Xen control structures to pass data back and forth with DomUs. In addition, the privileged guest requires multiplex access to underlying resources—such as the processor, memory, network and storage—on behalf of DomUs.

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