Linux in a Nutshell, 6th Edition

Conceptual Overview

All of the virtualization programs we cover in this chapter provide full system virtualization. They use a software layer to present a virtualized hardware system. This layer, called the virtual machine monitor (VMM) or hypervisor, makes it possible to run multiple operating systems simultaneously. The operating system at the base—running directly on the hardware and interacting with it directly—is called the host, and the software instances that run on top of it—interacting only indirectly with hardware—are called the guests.

Virtualization isn’t a new topic. As a means to divide a computer’s resources into multiple running environments, virtualization has been around since the 60s. Back then, IBM used it to partition mainframe computers into separate virtual machines, allowing it to run multiple applications and processes simultaneously. Although low-cost PC hardware generally replaced mainframe hardware virtualization in the 80s and 90s, virtual machines were still commonly used to provide a virtualized environment for a process: e.g., creating a security sandbox or providing greater portability. The Java Virtual Machine (JVM) is a good example of process virtualization. This kind of virtualization also includes tools like chroot and virtfs.

Another popular type of virtualization creates a new instance of the same operating system on top of the host. This technology is often called containers. It typically includes such projects as Solaris Zones and User Mode ...