Introduction to Virtualization - Oracle Solaris 10 System Virtualization Essentials - page 27

As described in detail later in this chapter, hardware partitions offer the most
isolation but the least flexibility. This arrangement is appropriate for business-
critical workloads where service availability is the most important factor. Each
partition has complete control over its hardware. At the other end of the spectrum,
operating system virtualization (OSV) offers the most flexible configurations but
the least isolation between the VEs, which are often called containers . Containers
also provide the best scalability and have demonstrated the highest virtualization
density. OSV is also discussed later in this chapter.
Between those two extremes, the virtual machines model creates the illusion
that many computers are present, using one computer and a layer of firmware and
software. That layer is the hypervisor, which provides multiplexed access from
each operating system instance to the shared hardware. It also provides the abil-
ity to install, start, and stop each of those instances. Note that two types of VM
implementations are possible: A Type 1 hypervisor runs directly on the hardware,
while a Type 2 hypervisor runs on an operating system. Both types of hypervisors
are discussed later in more detail.
Some of these virtualization models can be combined in one system, as shown
in Figure 1.16. For example, one virtual machine can run an OS that also sup-
ports OSV. You can use layered virtualization to benefit from the strengths of each
type. This strategy does, however, add complexity, which is most noticeable when
troubleshooting problems.
Figure 1.16 Multilayered Virtualization
The next few sections of this chapter describe each of the three virtualization
categories shown in Figure 1.15: hardware partitions, virtual machines, and op-
erating system virtualization. The descriptions are generic, discussing factors