Information Technology Reference
In-Depth Information
The first type of virtualization, and one of the most popular, is virtual machines.
This model provides the illusion that many independent computers are present in
the system, each running a copy of an OS. Each of these VEs is called a
virtual ma-
chine.
Software or firmware, or a combination of both, manages the OS instances
and provides multiplexed access to the hardware. This supporting layer, which
acts as the hypervisor, gives this model its flexibility but adds a certain amount of
performance overhead while it performs its tasks.
Failure isolation of hypervisors varies with implementation. Each shared re-
source is a single point of failure, including the hypervisor itself.
Most hypervisors provide virtual machines that mimic the physical hardware.
A few of them emulate a completely different hardware architecture. Some of
these are used to develop new hardware, simulating the hardware in software or
testing software that will run on the hardware. Others are used to run software
compiled for a CPU architecture that is not available or is not economical to con-
tinue operating.
1.2.2.1 Type 1 Hypervisors
A Type 1 hypervisor comprises software or firmware that runs directly on the
computer's hardware. It typically has components found in a complete operating
system, including device drivers. Some implementations offer the ability to assign
a set or quantity of physical CPUs or CPU cores to a specific VE. Other imple-
mentations use a scheduler to give each operating system instance a time slice on
the CPU(s). Some versions offer both. Each VE appears to be its own computer,
and each appears to have complete access to the hardware resources assigned to
it, including I/O devices. Although hypervisors also provide shared access to I/O
devices, this capability inflicts a larger performance penalty.
These hypervisors implement a small feature set designed exclusively for host-
ing virtual machines. When the system starts, the hypervisor is placed into the
main system RAM or specific area of reserved memory; in some architectures,
additional elements reside in firmware, hardware, and BIOS. The hypervisor may
make use of or require specialized hardware-assist technology to decrease hyper-
visor overhead and increase performance and reliability.
A Type 1 hypervisor is a small specialized environment designed specifically
for the task of hosting virtual machines. This model has several advantages over
Type 2 hypervisors—namely, simplicity of design, a smaller attack surface, and
less code to analyze for security validation. The primary disadvantages of Type
1 hypervisors are that they require more coding and they do not allow a base
operating system to run any applications with native performance. Also, they can-
not freely leverage services provided by a host OS. Even mundane features such
as a management interface or file system may need to be built “from scratch” for
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