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Contents

  1. Introduction
  2. Installation
  3. Creating a new guest
  4. Setting up bridge networking
    1. Between VM guests
    2. Between VM host and guests
    3. Between VM host, guests and the world
  5. Managing VMs from the command-line
  6. Managing VM guests with a GUI
  7. Automatic guest management on host shutdown/startup
  8. Performance Tuning
    1. CPU
    2. Disk I/O
    3. Network I/O
    4. Memory
  9. Migrating guests to a Debian host
    1. Migrating guests from RHEL/CentOS 5.x
  10. Troubleshooting
  11. See also
  12. External links

Introduction

KVM is a full virtualization solution for Linux on x86 (64-bit included) hardware containing virtualization extensions (Intel VT or AMD-V). It consists of a loadable kernel module, kvm.ko, that provides the core virtualization infrastructure and a processor specific module, kvm-intel.ko or kvm-amd.ko.

In Debian, Xen and VirtualBox are alternatives to KVM.

Installation

It is possible to install only QEMU and KVM for a very minimal setup, but most users will also want libvirt-daemon or even virt-manager for a GUI. For Debian/stretch, jessie-backports, and newer: 

# apt install qemu-kvm libvirt-clients libvirt-daemon-system

For jessie and older: 

# apt-get install qemu-kvm libvirt-bin

The daemon libvirt-bin daemon will start automatically at boot time and load the appropriate kvm modules, kvm-amd or kvm-intel, which are shipped with the Linux kernel Debian package. If you intend to create VMs from the command-line, install virtinst.

In order to be able to manage virtual machines as regular user, that user needs to be added to some groups. For Debian/stretch, jessie-backports, and newer: 

# adduser <youruser> libvirt
# adduser <youruser> libvirt-qemu

For jessie and older: 

# adduser <youruser> kvm
# adduser <youruser> libvirt

You should then be able to list your domains: 

# virsh list --all

libvirt defaults to qemu:///session for non-root. So from <youruser> you'll need to do: 

$ virsh --connect qemu:///system list --all

You can use LIBVIRT_DEFAULT_URI to change this.

Creating a new guest

The easiest way to create and manage VM guest is using GUI application Virtual Machine Manager virt-manager.

In alternative, you may create VM guest via command line. Below is example to create a Squeeze guest with name squeeze-amd64: 

virt-install --virt-type kvm --name squeeze-amd64 --memory 512 --cdrom ~/iso/Debian/cdimage.debian.org_mirror_cdimage_archive_6.0.10_live_amd64_iso_hybrid_debian_live_6.0.10_amd64_gnome_desktop.iso --disk size=4 --os-variant debiansqueeze

Since the guest has no network connection yet, you will need to use the GUI virt-viewer to complete the install.

You can avoid pulling the ISO by using the --location option. To obtain text console for the installation you can also provide --extra-args "console=ttyS0": 

virt-install --virt-type kvm --name squeeze-amd64 \
--location http://httpredir.debian.org/debian/dists/squeeze/main/installer-amd64/ \
--extra-args "console=ttyS0" -v --os-variant debiansqueeze \
--disk size=4 --memory 512

For a fully automated install look into preseed or debootstrap.

Setting up bridge networking

Between VM guests

By default, QEMU uses macvtap in VEPA mode to provide NAT internet access or bridged access with other guest. Unfortunately, this setup could not let the host to communicate with any guests.

Between VM host and guests

To let communications between VM host and VM guests, you may setup a macvlan bridge on top of a dummy interface similar as below. After the configuration, you can set using interface dummy0 (macvtap) in bridged mode as the network configuration in VM guests configuration. 

modprobe dummy
ip link add dummy0 type dummy
ip link add link dummy0 macvlan0 type macvlan mode bridge
ifconfig dummy0 up
ifconfig macvlan0 192.168.1.2 broadcast 192.168.1.255 netmask 255.255.255.0 up

Between VM host, guests and the world

In order to let communications between host, guests and outside world, you may set up a bridge and as described at QEMU page.

For example, you may modify network configuration file /etc/network/interfaces for setup ethernet interface eth0 to a bridge interface br0 similar as below. After the configuration, you can set using Bridge Interface br0 as the network connection in VM guests configuration. 

auto lo
iface lo inet loopback

# The primary network interface
auto eth0

#make sure we don't get addresses on our raw device
iface eth0 inet manual
iface eth0 inet6 manual

#set up bridge and give it a static ip
auto br0
iface br0 inet static
        address 192.168.1.2
        netmask 255.255.255.0
        network 192.168.1.0
        broadcast 192.168.1.255
        gateway 192.168.1.1
        bridge_ports eth0
        bridge_stp off
        bridge_fd 0
        bridge_maxwait 0
        dns-nameservers 8.8.8.8

#allow autoconf for ipv6
iface br0 inet6 auto
        accept_ra 1

Managing VMs from the command-line

You can then use the virsh(1) command to start and stop virtual machines. VMs can be generated using virtinst. For more details see the libvirt page. Virtual machines can also be controlled using the kvm command in a similar fashion to QEMU. Below are some frequently used commands:

Start a configured VM guest "VMGUEST": 

# virsh start VMGUEST

Notify the VM guest "VMGUEST" to graceful shutdown: 

# virsh shutdown VMGUEST

Force the VM guest "VMGUEST" to shutdown in case it is hanged, i.e. graceful shutdown not work: 

# virsh destroy VMGUEST

Managing VM guests with a GUI

On the other hand, if you want to use a graphical UI to manage the VMs, you can use the Virtual Machine Manager virt-manager.

Automatic guest management on host shutdown/startup

Guest behavior on host shutdown/startup is configured in /etc/default/libvirt-guests.

This file specifies whether guests should be shutdown or suspended, if they should be restarted on host startup, etc.

First parameter defines where to find running guests. For instance: 

# URIs to check for running guests
# example: URIS='default xen:/// vbox+tcp://host/system lxc:///'
URIS=qemu:///system

Performance Tuning

Below are some options which can improve performance of VM guests.

CPU

Disk I/O

Disk I/O is usually the bottleneck of performance due to its characteristics. Unlike CPU and RAM, VM host may not allocate a dedicated storage hardware for a VM. Worse, disk is the slowest component between them. There is two types of disk bottleneck, throughput and access time. A modern harddisk can perform 100MB/s throughput which is sufficient for most of the systems. While a modern harddisk can only provides around 60 transactions per seconds (tps).

For VM Host, you can benchmark different disk I/O parameters to get the best tps for your disk. Below is an example of disk tuning and benchmarking using fio:

For Windows VM guests, you may wish to switch between the slow but cross-platform Windows built-in IDE driver or fast but KVM specific VirtIO driver. As a result, the installation method for Windows VM guest provided below is a little bit complicated while provides a way to install both driver and use one for your needs. Under virt-manager:

Network I/O

Using virt-manager:

Memory

Migrating guests to a Debian host

Migrating guests from RHEL/CentOS 5.x

There are a few minor things in guest XML configuration files (/etc/libvirt/qemu/*.xml you need to modify:

In other words, the relevant sections should look something like this: 

  <os>
    <type arch='x86_64' machine='pc'>hvm</type>

  --- snip ---

  <devices>
    <emulator>/usr/bin/kvm</emulator>

If you had configured a bridge network on the CentOS host, please refer to this wiki article on how to make it work on Debian.

Troubleshooting

No network bridge available

virt-manager uses a virtual network for its guests, by default this is routed to 192.168.122.0/24 and you should see this by typing ip route as root.

If this route is not present in the kernel routing table then the guests will fail to connect and you will not be able to complete a guest creation.

Fixing this is simple, open up virt-manager and go to "Edit" -> "Host details" -> "Virtual networks" tab. From there you may create a virtual network of your own or attempt to fix the default one. Usually the problem exists where the default network is not started.

cannot create bridge 'virbr0': File exists:

To solve this probelm you may remove the virbr0 by running: 

brctl delbr virbr0

Open virt-manager and go to "Edit" -> "Host details" -> "Virtual networks" start the default network.

You can check the netstatus 

virsh net-list --all

Optionally, you can use bridge network BridgeNetworkConnections

Windows guest frequently hang or BSOD

Some Windows guest using some high-end N-way CPU may found frequently hang or BSOD, this is a known kernel bug while unfortunately not fixed in Jessie (TBC in Stretch). Below workaround can be applied by adding a section <hyperv>...</hyperv> in the guest configuration via command virsh edit GUESTNAME: 

<domain ...>
  ...
  <features>
    ...
    <hyperv>
      <relaxed state='on'/>
    </hyperv>
  </features>
  ...

See also

External links

Please, add links to external documentation. This is not a place for links to non-free commercial products.

CategorySystemAdministration