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advantages apply in server environments: Some applications on a server may run
“first level” directly on the OS, whereas other applications are hosted in virtual
machines, perhaps to provide increased isolation for security purposes, or to host
a different OS version without the disruption of installing a Type 1 hypervisor on
the bare metal. These advantages can be compelling enough to compensate for the
memory footprint of the hosting OS and the potential performance penalty of the
hypervisor. It is sometimes assumed that a Type 2 hypervisor is inherently less
efficient than a Type 1 hypervisor, but this need not be the case. Further, a Type 2
hypervisor may benefit from scalability and performance provided by the underly-
ing OS that would be challenging to replicate in a Type 1 hypervisor.
Examples of Type 2 hypervisors include Oracle VM VirtualBox, VMware Server,
VMware Fusion, Parallel Inc. Parallels Workstation, and Microsoft Windows
Virtual PC.
1.2.2.4 Full Virtualization and Paravirtualization
Another distinction between different forms of hypervisor-based virtualization is
whether full virtualization or paravirtualization is used. When full virtualization
is used, the hypervisor creates virtual machines that are architecturally consistent
with the “bare iron” physical machine. With paravirtualization, the hypervisor
provides software interfaces that virtual machines can use to communicate with
the hypervisor for requesting services and receiving event notifications.
The advantage of full virtualization is that unmodified operating systems can
be run as guests, simplifying migration and technology adoption, albeit at the cost
of requiring the hypervisor to implement all platform details. This approach can
create substantial overhead depending on the platform, especially for I/O, timer,
and memory management.
Paravirtualization offers the opportunity to optimize performance by provid-
ing more efficient software interfaces to these and other functions. It can include
cooperative processing between guest and hypervisor for memory management
(e.g., memory ballooning, shared pages for I/O buffers), shortened I/O path length
(e.g., device drivers making direct calls to the hypervisor or combining multiple
I/O requests into a single request), clock skew management, and other optimiza-
tions. The disadvantage of paravirtualization is that it requires source code to
port the guest OS onto the virtualized platform, or at least the ability to add
optimized devices drivers.
Examples of paravirtualization include Xen implementations such as Oracle
VM Server for x86 and Citrix XenServer, Oracle VM Server for SPARC, the guest/
host additions in Oracle VM VirtualBox and VMware ESX, and the Conversational
Monitor System (CMS) under VM/370.
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