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Virtualization

Table of contents​

  1. Virtualization concepts
  2. KVM and QEMU
  3. Management with libvirt
  4. virsh - Management CLI
  5. Network and storage
  6. Practical exercises

1 - Virtualization concepts​

Types of virtualization​

TypeExamplesUsage
Type 1VMware ESXi, KVM, Hyper-VProduction
Type 2VirtualBox, VMware WorkstationDesktop

KVM vs Containers​

AspectKVM (VMs)Containers
IsolationComplete (separate kernel)Shared kernel
OverheadHigherMinimal
BootMinutesSeconds
SecurityStrong isolationWeaker isolation
Use caseDifferent OSes, strong isolationMicroservices, scaling

Hardware requirements​

# Vérifier le support de virtualisation
grep -E 'vmx|svm' /proc/cpuinfo

# vmx = Intel VT-x
# svm = AMD-V

# Charger les modules
lsmod | grep kvm
# kvm_intel ou kvm_amd

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2 - KVM and QEMU​

Installation​

# Debian/Ubuntu
apt install qemu-kvm libvirt-daemon-system \
    libvirt-clients bridge-utils virtinst

# RHEL/CentOS
yum install qemu-kvm libvirt virt-install

# Démarrer libvirtd
systemctl enable --now libvirtd

# Ajouter l'utilisateur au groupe
usermod -aG libvirt $USER

Verification​

# Vérifier KVM
kvm-ok
# INFO: /dev/kvm exists
# KVM acceleration can be used

# Vérifier libvirt
virsh list --all

# Vérifier la connexion
virsh uri
# qemu:///system

Create a VM with virt-install​

# Depuis une ISO
virt-install \
    --name ubuntu22 \
    --ram 2048 \
    --vcpus 2 \
    --disk path=/var/lib/libvirt/images/ubuntu22.qcow2,size=20 \
    --os-variant ubuntu22.04 \
    --network bridge=virbr0 \
    --graphics vnc,listen=0.0.0.0 \
    --cdrom /var/lib/libvirt/boot/ubuntu-22.04-server.iso

# Depuis une image cloud
virt-install \
    --name cloud-vm \
    --ram 2048 \
    --vcpus 2 \
    --import \
    --disk /var/lib/libvirt/images/cloud-vm.qcow2 \
    --os-variant ubuntu22.04 \
    --network bridge=virbr0 \
    --noautoconsole

Disk formats​

FormatAdvantagesDrawbacks
qcow2Snapshots, compression, thin provisioningSlightly slower
rawMaximum performanceNo snapshots
# Convertir
qemu-img convert -O qcow2 disk.raw disk.qcow2

# Infos sur un disque
qemu-img info disk.qcow2

# Redimensionner
qemu-img resize disk.qcow2 +10G

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3 - Management with libvirt​

libvirt architecture​

Configuration files​

# Configuration libvirt
/etc/libvirt/libvirtd.conf

# Définitions des VMs (XML)
/etc/libvirt/qemu/

# Images disque par défaut
/var/lib/libvirt/images/

# Logs
/var/log/libvirt/

XML definition of a VM​

<!-- /etc/libvirt/qemu/myvm.xml -->
<domain type='kvm'>
  <name>myvm</name>
  <memory unit='MiB'>2048</memory>
  <vcpu>2</vcpu>
  <os>
    <type arch='x86_64'>hvm</type>
    <boot dev='hd'/>
  </os>
  <devices>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/myvm.qcow2'/>
      <target dev='vda' bus='virtio'/>
    </disk>
    <interface type='network'>
      <source network='default'/>
      <model type='virtio'/>
    </interface>
    <graphics type='vnc' port='-1'/>
  </devices>
</domain>

virt-manager (GUI)​

# Installer
apt install virt-manager

# Lancer
virt-manager

# Connexion distante via SSH
virt-manager -c qemu+ssh://user@host/system

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4 - virsh - Management CLI​

Basic commands​

# Lister les VMs
virsh list --all

# Démarrer/arrêter
virsh start myvm
virsh shutdown myvm    # Graceful
virsh destroy myvm     # Force

# Redémarrer
virsh reboot myvm

# Pause/Resume
virsh suspend myvm
virsh resume myvm

VM management​

# Infos sur une VM
virsh dominfo myvm

# Console
virsh console myvm

# Afficher la config XML
virsh dumpxml myvm

# Éditer la config
virsh edit myvm

# Définir/Supprimer
virsh define myvm.xml
virsh undefine myvm
virsh undefine myvm --remove-all-storage

Snapshots​

# Créer un snapshot
virsh snapshot-create-as myvm snap1 "Description"

# Lister les snapshots
virsh snapshot-list myvm

# Revenir à un snapshot
virsh snapshot-revert myvm snap1

# Supprimer
virsh snapshot-delete myvm snap1

Hot resources​

# Ajouter de la RAM (si supporté)
virsh setmem myvm 4G --live

# Ajouter des vCPUs
virsh setvcpus myvm 4 --live

# Ajouter un disque
virsh attach-disk myvm /path/to/disk.qcow2 vdb --live

# Détacher
virsh detach-disk myvm vdb --live

Autostart​

# Activer le démarrage automatique
virsh autostart myvm

# Désactiver
virsh autostart --disable myvm

# Lister les VMs en autostart
virsh list --autostart

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5 - Network and storage​

Virtual networks​

# Lister les réseaux
virsh net-list --all

# Infos sur default
virsh net-info default
virsh net-dumpxml default

# Créer un réseau bridge
cat > br-network.xml << 'EOF'
<network>
  <name>br-internal</name>
  <forward mode='bridge'/>
  <bridge name='br0'/>
</network>
EOF

virsh net-define br-network.xml
virsh net-start br-internal
virsh net-autostart br-internal

Create a bridge on the host​

# /etc/network/interfaces (Debian)
auto br0
iface br0 inet dhcp
    bridge_ports eth0
    bridge_stp off
    bridge_fd 0

# Ou avec netplan (Ubuntu)
# /etc/netplan/01-bridge.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: no
  bridges:
    br0:
      interfaces: [eth0]
      dhcp4: yes

Storage pools​

# Lister les pools
virsh pool-list --all

# Créer un pool de répertoire
virsh pool-define-as mypool dir --target /data/vms
virsh pool-build mypool
virsh pool-start mypool
virsh pool-autostart mypool

# Créer un volume dans le pool
virsh vol-create-as mypool disk1.qcow2 20G --format qcow2

# Lister les volumes
virsh vol-list mypool

VM migration​

# Migration live (nécessite stockage partagé)
virsh migrate --live myvm qemu+ssh://dest-host/system

# Migration avec copie du disque
virsh migrate --live --copy-storage-all myvm qemu+ssh://dest-host/system

# Offline migration
virsh migrate --offline myvm qemu+ssh://dest-host/system

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6 - Practical exercises​

Exercise 1: Create a VM​

Create a VM with 2 vCPUs, 2GB RAM and a 10GB disk:

Solution
# Créer le disque
qemu-img create -f qcow2 /var/lib/libvirt/images/test-vm.qcow2 10G

# Créer la VM
virt-install \
    --name test-vm \
    --ram 2048 \
    --vcpus 2 \
    --disk path=/var/lib/libvirt/images/test-vm.qcow2 \
    --os-variant generic \
    --network network=default \
    --graphics vnc \
    --cdrom /path/to/iso

Exercise 2: Snapshot and restore​

Create a snapshot, modify the VM, then restore it:

Solution
# Créer le snapshot
virsh snapshot-create-as test-vm clean-state "État initial"

# Modifier la VM (ex: supprimer des fichiers)

# Restaurer
virsh snapshot-revert test-vm clean-state

# Vérifier
virsh snapshot-list test-vm

Quiz​

Q1. Which command displays the XML configuration of a VM?

Answer

virsh dumpxml nom-vm

Q2. Which disk format supports snapshots?

Answer

qcow2

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Key takeaways​

  • KVM = Linux hypervisor built into the kernel
  • libvirt = unified management API
  • virsh = CLI to manage VMs
  • qcow2 = format with snapshots and thin provisioning
  • virsh list --all to see all VMs
  • Use bridges for VM networking
  • Snapshots let you save the state

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