Installation guide

Chapter 1.
1
Introduction
This chapter introduces various virtualization technologies, applications and features and explains how
they work. The purpose of this chapter is to assist Red Hat Enterprise Linux users in understanding
the basics of virtualization.
1.1. What is virtualization?
Virtualization is a broad computing term for running software, usually operating systems, concurrently
and isolated from other programs on one system. Most existing implementations of virtualization use a
hypervisor, a software layer that controls hardware and provides guest operating systems with access
to underlying hardware. The hypervisor allows multiple operating systems to run on the same physical
system by offering virtualized hardware to the guest operating system. There are various methods for
virtualizing operating systems:
• Hardware-assisted virtualization is the technique used for full virtualization with KVM.
• Para-virtualization is a technique used by Xen to run Linux guests.
• Software virtualization or emulation. Software virtualization uses binary translation and other
emulation techniques to run unmodified operating systems. Software virtualization is significantly
slower than hardware-assisted virtualization or para-virtualization. Software virtualization, using
QEMU without KVM, is unsupported by Red Hat Enterprise Linux.
1.2. KVM and virtualization in Red Hat Enterprise Linux
What is KVM?
(KVM) is a Full virtualization solution for Linux on AMD64 and Intel 64 hardware. KVM is a Linux
kernel module built for the standard Red Hat Enterprise Linux 6 kernel. KVM can run multiple,
unmodified virtualized guest Windows and Linux operating systems.The KVM hypervisor in Red Hat
Enterprise Linux is managed with the libvirt API and tools built for libvirt, virt-manager and virsh.
Virtualized guests are run as Linux processes and threads which are controlled by these modules.
Red Hat Enterprise Linux KVM hypervisors can be managed by the Red Hat Enterprise Virtualization
Manager as an alternative to the virsh and virt-manager tools.
The kvm package also contains Linux kernel modules which manage devices, memory and
management APIs for the Hypervisor module itself.
This book covers virtualization topics for Red Hat Enterprise Linux 6. The Kernel based Virtual
Machine (KVM) hypervisor is provided with Red Hat Enterprise Linux. The KVM hypervisor supports
Full virtualization.
Overcommitting
The KVM hypervisor supports overcommitting virtualized CPUs and memory. Overcommitting means
allocating more virtualized CPUs or memory than the available resources on the system. CPU
overcommitting allows virtualized guests to run on fewer servers and in higher densities. Memory
overcommitting allows hosts to utilize memory and virtual memory to increase guest densities.
For more information on overcommitting with KVM, refer to Chapter 20, Overcommitting with KVM.