Kvm/Qemu

Chapter 59: KVM/QEMU Virtualization - Deep Dive

Mastering Kernel-Based Virtual Machine Technology

Why This Matters in DevOps/SRE

KVM (Kernel-based Virtual Machine) is enterprise-grade virtualization - essential for running VMs on Linux servers, private cloud environments, and infrastructure-as-a-service. Unlike containers, VMs provide full OS isolation.

┌─────────────────────────────────────────────────────────────────────────────┐
│                    KVM IN DEVOPS                                           │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                             │
│  ┌─────────────────────────────────────────────────────────────────────┐    │
│  │                    USE CASES                                              │    │
│  │                                                                       │    │
│  │   • Private cloud - Self-hosted IaaS                                  │    │
│  │   • Legacy apps - Run older OS versions                              │    │
│  │   • Windows workloads - Windows VMs on Linux                         │    │
│  │   • Development - Isolated testing environments                      │    │
│  │   • Security - Complete OS isolation from host                       │    │
│  │                                                                       │    │
│  └─────────────────────────────────────────────────────────────────────┘    │
│                                                                             │
│  ┌─────────────────────────────────────────────────────────────────────┐    │
│  │                    KEY CONCEPTS                                           │    │
│  │                                                                       │    │
│  │   • KVM: Linux kernel module for hardware virtualization              │    │
│  │   • QEMU: Emulator for disk/memory management                        │    │
│  │   • libvirt: Management API (virsh, virt-manager)                   │    │
│  │   • virt-install: CLI for creating VMs                               │    │
│  │                                                                       │    │
│  └─────────────────────────────────────────────────────────────────────┘    │
│                                                                             │
│  ┌─────────────────────────────────────────────────────────────────────┐    │
│  │                    PRODUCTION TASKS                                       │    │
│  │                                                                       │    │
│  │   • Live migration: Move VMs between hosts without downtime           │    │
│  │   • Snapshots: Point-in-time backups                                 │    │
│  │   • Resource limits: CPU/memory/IO limits                           │    │
│  │   • Storage pools: Centralized storage management                    │    │
│  │                                                                       │    │
│  └─────────────────────────────────────────────────────────────────────┘    │
│                                                                             │
└─────────────────────────────────────────────────────────────────────────────┘

59.1 Understanding Virtualization

What is Virtualization?

Virtualization is a technology that allows multiple isolated virtual machines to run on a single physical server. KVM (Kernel-based Virtual Machine) turns the Linux kernel into a hypervisor.

                     Virtualization Types
+------------------------------------------------------------------+
|                                                                   |
|    Full Virtualization                                             |
|    +----------------------------------------------------------+   |
|    |  - Complete emulation of hardware                         |   |
|    |  - Guest OS doesn't know it's virtualized               |   |
|    |  - Uses: Legacy OS, OS testing                          |   |
|    |  - Examples: VMware, QEMU (no KVM)                    |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    Paravirtualization                                             |
|    +----------------------------------------------------------+   |
|    |  - Guest OS is aware of virtualization                 |   |
|    |  - Uses hypervisor APIs for I/O                        |   |
|    |  - Better performance than full virtualization         |   |
|    |  - Examples: Xen, early KVM                            |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    Hardware-Assisted Virtualization (HVM)                          |
|    +----------------------------------------------------------+   |
|    |  - CPU provides hardware support (Intel VT-x, AMD-V)   |   |
|    |  - Nearly native performance                           |   |
|    |  - Can run unmodified OS                               |   |
|    |  - Examples: KVM, VMware, Hyper-V                    |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    Container Virtualization                                        |
|    +----------------------------------------------------------+   |
|    |  - OS-level virtualization                             |   |
|    |  - Containers share host kernel                         |   |
|    |  - Lightweight, fast to start                         |   |
|    |  - Examples: Docker, LXC, Podman                      |   |
|    +----------------------------------------------------------+   |
|                                                                   |
+------------------------------------------------------------------+

KVM Architecture

                     KVM Architecture
+------------------------------------------------------------------+
|                                                                   |
|                        QEMU/KVM Stack                                |
|                                                                   |
|    +-------------------------------------------------------------+ |
|    |              libvirt (Management API)                        | |
|    |  (virsh, virt-manager, OpenStack, etc.)                    | |
|    +-----------------------------+-------------------------------+ |
|                                  |                                |
|                                  v                                |
|    +-----------------------------+-------------------------------+ |
|    |              libvirt daemon (libvirtd)                     | |
|    +-----------------------------+-------------------------------+ |
|                                  |                                |
|                                  v                                |
|    +-------------------------------------------------------------+ |
|    |                    QEMU Emulator                            | |
|    |  - Emulates hardware devices                              | |
|    |  - Handles I/O operations                                 | |
|    |  - Process: qemu-system-x86_64                          | |
|    +-----------------------------+-------------------------------+ |
|                                  |                                |
|                                  v                                |
|    +-----------------------------+-------------------------------+ |
|    |              KVM Kernel Module (kvm.ko, kvm-intel.ko)     | |
|    +-----------------------------+-------------------------------+ |
|                                  |                                |
|                                  v                                |
|    +-----------------------------+-------------------------------+ |
|    |                    Linux Kernel                            | |
|    |  - CPU scheduling                                        | |
|    |  - Memory management                                     | |
|    |  - Device drivers                                       | |
|    +-----------------------------+-------------------------------+ |
|                                                                   |
|                              |                                      |
|                              v                                      |
|                    Physical Hardware                                |
|                                                                   |
+------------------------------------------------------------------+

59.2 KVM Installation and Setup

Installing KVM

# =============================================================================
# ARCH LINUX
# =============================================================================

# Install packages
sudo pacman -S \
    qemu \
    libvirt \
    virt-manager \
    bridge-utils \
    dnsmasq \
    iptables \
    ovmf \
    edk2-ovmf

# Enable nested virtualization (if needed)
sudo modprobe kvm_intel nested=1

# Verify KVM
kvm-ok

# =============================================================================
# UBUNTU/DEBIAN
# =============================================================================

# Install packages
sudo apt update
sudo apt install \
    qemu-kvm \
    libvirt-daemon-system \
    libvirt-clients \
    bridge-utils \
    virt-manager

# Add user to groups
sudo usermod -aG libvirt $USER
sudo usermod -aG kvm $USER

# Start libvirtd
sudo systemctl enable --now libvirtd

# =============================================================================
# VERIFY INSTALLATION
# =============================================================================

# Check modules
lsmod | grep kvm

# Check KVM devices
ls -la /dev/kvm

# Verify libvirtd
sudo systemctl status libvirtd
virsh list --all

# Test with guest
virt-install --name test --os-variant detect --ram 1024 --disk path=/var/lib/libvirt/images/test.qcow2 --pxe --dry-run

KVM Network Setup

# =============================================================================
# BRIDGE NETWORK
# =============================================================================

# Create bridge (using nmcli)
nmcli con add type bridge ifname br0
nmcli con add type ethernet ifname eth0 master br0
nmcli con down eth0
nmcli con up br0

# Using brctl (deprecated but still works)
sudo brctl addbr br0
sudo brctl addif br0 eth0
sudo brctl show

# =============================================================================
# LIBVIRT NETWORK
# =============================================================================

# Create network XML /tmp/bridge-network.xml
<network>
  <name>bridge-network</name>
  <forward mode='bridge'/>
  <bridge name='br0'/>
</network>

# Define and start network
virsh net-define /tmp/bridge-network.xml
virsh net-start bridge-network
virsh net-autostart bridge-network

# =============================================================================
# NAT NETWORK (DEFAULT)
# =============================================================================

# Default NAT network comes with libvirt
virsh net-list
virsh net-start default

59.3 Virtual Machine Management

Creating VMs with virt-install

# =============================================================================
# BASIC VM CREATION
# =============================================================================

# Create VM with ISO
virt-install \
    --name=webserver \
    --vcpu=2 \
    --memory=2048 \
    --disk path=/var/lib/libvirt/images/webserver.qcow2,size=20 \
    --os-variant=ubuntu22.04 \
    --cdrom=/path/to/ubuntu.iso \
    --network network=default \
    --graphics=vnc

# Create VM with PXE boot
virt-install \
    --name=pxe-server \
    --vcpu=2 \
    --memory=2048 \
    --disk path=/var/lib/libvirt/images/pxe.qcow2,size=10 \
    --os-variant=ubuntu22.04 \
    --pxe \
    --network network=default

# Create VM with cloud-init
virt-install \
    --name=cloud-vm \
    --vcpu=2 \
    --memory=4096 \
    --disk path=/var/lib/libvirt/images/cloud.qcow2,size=20 \
    --os-variant=ubuntu22.04 \
    --cloud-init root-password-generate=on \
    --network network=default

# Create from existing image
virt-install \
    --name=imported \
    --vcpu=2 \
    --memory=2048 \
    --disk path=/var/lib/libvirt/images/existing.qcow2 \
    --import \
    --os-variant=auto

# =============================================================================
# CONSOLE ACCESS
# =============================================================================

# Connect to VM console
virt-viewer webserver

# Connect via serial console
virsh console webserver

# Exit serial console: Ctrl+]

# =============================================================================
# VM CLONING
# =============================================================================

# Clone VM
virt-clone --original=webserver --name=webserver-clone --auto-clone

# Clone to new storage
virt-clone --original=webserver \
    --name=webserver-clone \
    --file=/var/lib/libvirt/images/clone.qcow2

Managing VMs with virsh

# =============================================================================
# VM LIFECYCLE
# =============================================================================

# List running VMs
virsh list

# List all VMs
virsh list --all

# Start VM
virsh start webserver

# Stop VM (graceful)
virsh shutdown webserver

# Force stop VM
virsh destroy webserver

# Reboot VM
virsh reboot webserver

# Pause/Resume
virsh suspend webserver
virsh resume webserver

# Autostart VM
virsh autostart webserver
virsh autostart --disable webserver

# =============================================================================
# VM INFORMATION
# =============================================================================

# VM details
virsh dominfo webserver

# VM state
virsh domstate webserver

# List VCPUs
virsh vcpucount webserver

# Get VNC port
virsh vncdisplay webserver

# =============================================================================
# VM MODIFICATION
# =============================================================================

# Resize memory (online)
virsh setmem webserver 4096 --current
virsh setmem webserver 4096 --config  # persistent

# Resize VCPUs (online)
virsh setvcpus webserver 4 --current
virsh setvcpus webserver 4 --config  # persistent

# Attach disk
virsh attach-disk webserver /var/lib/libvirt/images/newdisk.qcow2 vdb --persistent

# Detach disk
virsh detach-disk webserver vdb --persistent

# Attach network
virsh attach-interface webserver --type network --source default --persistent

# =============================================================================
# VM DELETION
# =============================================================================

# Undefine VM (remove definition)
virsh undefine webserver

# Undefine with storage
virsh undefine webserver --remove-all-storage

# Delete snapshots before undefine
virsh undefine webserver --snapshots-metadata

59.4 Storage Management

KVM Storage Pools

                     KVM Storage Types
+------------------------------------------------------------------+
|                                                                   |
|    Directory (dir)                                               |
|    +----------------------------------------------------------+   |
|    |  - File-based storage (qcow2, raw)                       |   |
|    |  - Default for most setups                               |   |
|    |  - Easiest to manage                                    |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    LVM                                                            |
|    +----------------------------------------------------------+   |
|    |  - LVM volumes as VM disks                              |   |
|    |  - Better performance                                   |   |
|    |  - Requires LVM setup                                   |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    iSCSI                                                         |
|    +----------------------------------------------------------+   |
|    |  - Network storage                                       |   |
|    |  - For shared storage                                  |   |
|    |  - Requires iSCSI target                               |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    NFS                                                            |
|    +----------------------------------------------------------+   |
|    |  - Network file system                                  |   |
|    |  - For live migration                                  |   |
|    |  - Requires NFS server                                  |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    Ceph/RBD                                                       |
|    +----------------------------------------------------------+   |
|    |  - Distributed storage                                  |   |
|    |  - Highly available                                    |   |
|    |  - For production clusters                             |   |
|    +----------------------------------------------------------+   |
|                                                                   |
+------------------------------------------------------------------+

Storage Commands

# =============================================================================
# STORAGE POOLS
# =============================================================================

# List pools
virsh pool-list
virsh pool-list --all

# Create directory pool
virsh pool-define-as default dir - - - - /var/lib/libvirt/images
virsh pool-start default

# Create LVM pool
virsh pool-define-as vgpool logical - /dev/vg_kvm /dev/empty
virsh pool-start vgpool

# Pool info
virsh pool-info default

# Build pool
virsh pool-build default

# Refresh pool
virsh pool-refresh default

# Delete pool
virsh pool-destroy default
virsh pool-undefine default

# =============================================================================
# STORAGE VOLUMES
# =============================================================================

# List volumes
virsh vol-list default

# Create volume
virsh vol-create-as default vmdisk.qcow2 20G

# Create volume with specific format
virsh vol-create-as default vmdisk.raw 10G --format raw

# Clone volume
virsh vol-clone --pool default vmdisk.qcow2 vmdisk-clone.qcow2

# Resize volume
virsh vol-resize default/vmdisk.qcow2 40G

# Delete volume
virsh vol-delete default/vmdisk.qcow2

# =============================================================================
# DISK FORMATS
# =============================================================================

# qcow2 (Copy on Write) - Recommended
# - Snapshot support
# - Compression
# - Encryption
# - Thin provisioning

# raw - Fastest
# - No features
# - Full allocation

# qcow2 creation
qemu-img create -f qcow2 /var/lib/libvirt/images/vm.qcow2 20G

# Convert raw to qcow2
qemu-img convert -f raw -O qcow2 input.raw output.qcow2

# Convert to raw
qemu-img convert -f qcow2 -O raw input.qcow2 output.raw

# Resize disk
qemu-img resize vm.qcow2 +10G

# Check disk
qemu-img check vm.qcow2

59.5 Snapshots

Snapshot Management

# =============================================================================
# SNAPSHOT TYPES
# =============================================================================

# Internal snapshots
# - Stored within qcow2 file
# - Cannot live merge
# - Limited performance

# External snapshots
# - Backing chain
# - Can live merge
# - Better performance

# =============================================================================
# CREATE SNAPSHOT
# =============================================================================

# Create internal snapshot
virsh snapshot-create webserver
virsh snapshot-create-as webserver --name "before-update"

# Create external snapshot
virsh snapshot-create-as webserver \
    --name "backup-snapshot" \
    --disk-only \
    --diskspec vda,file=/var/lib/libvirt/images/webserver-backup.qcow2

# =============================================================================
# LIST SNAPSHOTS
# =============================================================================

# List all snapshots
virsh snapshot-list webserver

# Current snapshot
virsh snapshot-current webserver

# =============================================================================
# REVERT TO SNAPSHOT
# =============================================================================

# Revert to snapshot
virsh snapshot-revert webserver before-update

# =============================================================================
# DELETE SNAPSHOT
# =============================================================================

# Delete snapshot
virsh snapshot-delete webserver before-update

# =============================================================================
# MERGE SNAPSHOT
# =============================================================================

# Block commit (merge to base)
virsh blockcommit webserver vda --active --verbose --pivot

# Block pull (merge to top)
virsh blockpull webserver vda --verbose

59.6 Live Migration

VM Migration

                     Live Migration Types
+------------------------------------------------------------------+
|                                                                   |
|    Pre-copy Migration                                             |
|    +----------------------------------------------------------+   |
|    |  1. Initial transfer (memory pages)                      |   |
|    |  2.迭代 memory transfer (dirty pages)                    |   |
|    |  3. Stop source VM                                      |   |
|    |  4. Transfer remaining pages                            |   |
|    |  5. Start destination VM                                |   |
|    |                                                           |   |
|    |  - Minimal downtime                                     |   |
|    |  - Requires shared storage                              |   |
|    +----------------------------------------------------------+   |
|                                                                   |
|    Post-copy Migration                                            |
|    +----------------------------------------------------------+   |
|    |  1. Stop source VM                                      |   |
|    |  2. Transfer memory                                     |   |
|    |  3. Start destination VM                               |   |
|    |                                                           |   |
|    |  - Faster transfer                                      |   |
|    |  - Risk of source failure                               |   |
|    +----------------------------------------------------------+   |
|                                                                   |
+------------------------------------------------------------------+

Migration Commands

# =============================================================================
# LIVE MIGRATION
# =============================================================================

# Basic live migration
virsh migrate --live webserver qemu+ssh://dest-host/system

# With compression
virsh migrate --live --compress webserver qemu+ssh://dest-host/system

# With tunneled migration
virsh migrate --live --tunnelled webserver qemu+ssh://dest-host/system

# =============================================================================
# MIGRATION WITH STORAGE
# =============================================================================

# Migrate with non-shared storage
virsh migrate --live \
    --copy-storage-all \
    webserver \
    qemu+ssh://dest-host/system

# Migrate with incremental storage
virsh migrate --live \
    --copy-storage-incremental \
    webserver \
    qemu+ssh://dest-host/system

# =============================================================================
# MIGRATION OPTIONS
# =============================================================================

# Set maximum downtime
virsh migrate-setmaxdowntime webserver 1000

# Auto-converge (reduce memory during migration)
virsh migrate --live --auto-converge webserver qemu+ssh://dest-host/system

# Bandwidth limit (Mbps)
virsh migrate --live --bandwidth 100 webserver qemu+ssh://dest-host/system

# =============================================================================
# VERIFY MIGRATION
# =============================================================================

# Check migration status
virsh migrate-getspeed webserver

# List completed migrations
virsh migrate-compeleted webserver

59.7 Exam Tips

KVM vs QEMU: KVM = kernel module, QEMU = emulator
virt-install: Create VMs from command line
virsh: Main CLI for VM management
Storage: qcow2 supports snapshots, raw is faster
Networks: Bridge for external access, NAT for isolated
Live migration: Requires shared storage or —copy-storage-all
libvirt: API/daemon that powers KVM management
snapshots: Internal vs external, understand tradeoffs
Nested virtualization: Enable kvm_intel nested=1
Performance: Use virtio drivers for disk/network

Common Mistakes & Anti-Patterns

1. Not Using Virtio Drivers

WRONG:

# Using default IDE/SCSI drivers
# Poor disk/network performance

CORRECT:

# Use virtio for better performance
virt-install \
  --disk path=vm.qcow2,bus=virtio \
  --network bridge=br0,model=virtio

Why: Virtio provides ~3x better I/O performance than emulated drivers.

2. Not Setting Up Proper Networking

WRONG:

# Default NAT - VM not accessible from network
# Can't connect to services running in VM

CORRECT:

# Create bridge for external access
virsh net-edit default
# Or use host bridge
brctl addif br0 eth0

Why: NAT isolates VMs; bridge makes them first-class network citizens.

3. Using Raw Image Withoutthin Provisioning

WRONG:

# Pre-allocated raw - takes full disk space immediately
qemu-img create -f raw vm.img 100G

CORRECT:

# qcow2 with thin provisioning
qemu-img create -f qcow2 vm.qcow2 100G
# Only uses actual disk space

Why: qcow2 saves space and supports snapshots.

4. No Resource Limits

WRONG:

# VM can consume all host resources
# Host becomes unresponsive

CORRECT:

# Set CPU/memory limits
virsh setvcpus vm1 4 --maximum
virsh setmem vm1 8G --maximum

# Or in XML:
# <cpu mode='host-passthrough'>
#   <topology sockets='1' cores='4' threads='1'/>
# </cpu>

Why: Prevents VM from affecting other VMs or host.

5. Not Using Snapshots for Backups

WRONG:

# No snapshots - risky updates
# Can't rollback if something goes wrong

CORRECT:

# Create snapshot before risky changes
virsh snapshot-create-as vm1 --name "before-update"

# Revert if needed
virsh snapshot-revert vm1 before-update

# Delete after success
virsh snapshot-delete vm1 before-update

Why: Snapshots provide quick rollback capability.

Summary

In this chapter, you learned:

✅ Virtualization types and concepts
✅ KVM architecture
✅ KVM installation
✅ VM creation with virt-install
✅ VM management with virsh
✅ Storage pools and volumes
✅ Snapshots
✅ Live migration
✅ Performance tuning

Next Chapter

Chapter 57: Docker Fundamentals

Last Updated: February 2026