Virtualizations is a technology that allows us to abstract the hardware of a single computer (the CPU, memory, disk drivers, network interface cards, and so forth) into several different execution environments, thereby creating the illusion that each separate envirnment is running on its own private computer. These environments can be viewed as different individual operating systems(for example, Windows and UNIX) that may gbe running at the same time and may interact with each other. A user of a virtual machine can switch among the various operating systems in the same way a user can switch among the various operating systems in the same way a user can switch among the various processes runing concurrently in a single operating system.
Virtualization allows operating systems to run as application within other operating systems. At first blush, there seems to be littel reson for such functionality. But the virtualization industry is vast and growing. which is a testament to its utility and importance.
Broadly speaking, virtualization software is one members of a class that also includes emulation. Emulation, which involves simulating computer hardware in software, is typically used when the source CPU type is different from the target CPU type. For example, when Apple switched from the IBM Power CPU to the Intel x86 CPU for its desktop and laptop computers, it included an cmulation facility called "Rosetta," which allowed applications comiled for the IMB CPU to run on th eIntel CPU. That sme concept can be extended to allow an entire operating system written for one platform to run on another. Emulation comes at a heavy price, however. Every machine-level instruction that runs natively on the ousrce system must be translated to the equivalent function on the target system, frequently resulting in serveral target instructions. If the source and target CPUs have similar performance levels, the emulated code may run much more slowly than the native code.
With virtualization, in contrast, an operating system that is natively compiled for a particular CPU architecture runs within another operating system also native to that CPU. Virtualization first came about on IBM mainframes as a method for multiple users to run tasks concurrently. Running multiple virtual machines allowed (and still allows) many users to run tasks on a system designed for a single user. Lather, in response to problems with running multiple Microsoft Windows applications on the Intel x86 CPU, VMware created a new virtualization technology in the form of an application that ran on Windows. That application ran one or more guest copies of Windows or other native x86 operating systems, each running its own applications. (See Figure 1.16)
Windows was the host operating ssytem, and the VMware application was the virtual machine manager(VMM). The VMM runs the guest operating systems, manages their resource use, and protectes each guest from the others.
Even though modern operating systems are fully capable of running multiple applications reliably, the use of virtualization continues to grow. On laptops and desktops, a VMM allows athe user to install multiple operating systems for exploration or to run applications written for operating systems other than the native host. For example, an Apple laptop running macos on the x86 CPUcan run a Windows 10 guest to allow execution of Windows applications. Companies writing software for multiple operating systems can use virtualization to run all of those operating systems on a single physical server for development, testing, and debugging. Within data centers, virtualization has become a common method of executing and managing computing environments. VMMs like VMware ESXand Citrix XenServer no longer run on host operating systems but rather are the host operating systems, providing services and resource management to virtual machine processes.
With this text, we provide a Linux virtual machine that allows you to run Linux-as well as the development tools we provide-on your personnal system regardless of your host operating system. Full details of the features and implementation of virtualization can be found in Chapter 18.
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