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1.2.2.5 Relative Strengths of Hypervisors
Hypervisors typically represent a “middle ground” between the isolation of hard
partitions and the flexibility of OSV. The additional isolation of separate OS in-
stances compared to OSV allows for the consolidation of completely different oper-
ating systems. The hypervisor layer also provides a convenient point of separation
for VEs, thereby facilitating and simplifying VE mobility.
Some hypervisors offer optional CPU and I/O partitioning, which can reduce the
overhead of the hypervisor significantly. Of course, the scalability of this method is
limited by the number of CPUs and I/O buses. Systems with few CPUs must share
these resources among the VEs.
Hardware partitioning and virtual machine technologies share a common trait:
Each virtual environment contains an instance of an operating system. Most of
those technologies allow different operating systems to run concurrently.
In contrast, operating system virtualization (OSV) uses features of the operat-
ing system to create VEs that are not separate copies of an operating system. This
approach provides the appearance of an individual operating system instance for
each VE. Most OSV implementations provide the same OS type as the hosting
OS. Others, such as Oracle Solaris Containers, also have the ability to behave as
another operating system.
Figure 1.20 shows the relationship between the hardware, OS, and VEs when
using OSV.
Figure 1.20
Operating System Virtualization
We have already discussed the importance of isolation to virtualization. The
isolation between OSV VEs is just as important as the isolation noted in other
models. For OSV, this isolation is enforced by the OS kernel, rather than by a
hypervisor or hardware.
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