QEMU Disk Image and VM Management Guide

Introduction

QEMU is a powerful open-source emulator and virtualizer for running virtual machines (VMs) and managing disk images on Linux systems. It supports a wide range of architectures and provides tools for creating, manipulating, and running VMs with flexible configurations. This guide covers essential QEMU commands for disk image management and VM operations, with practical examples for system administrators and developers.

Prerequisites

  • QEMU installed on a Linux system (qemu-system-x86_64 --version to verify).
  • Root or sudo privileges for certain operations (e.g., installing QEMU or accessing KVM).
  • Basic understanding of Linux terminal commands and virtualization concepts.

Install QEMU on Ubuntu/Debian:

sudo apt update
sudo apt install qemu qemu-system qemu-utils

For KVM acceleration (recommended for better performance):

sudo apt install qemu-kvm

Verify KVM support:

kvm-ok

Disk Image Management

QEMU uses qemu-img to create, convert, and manage disk images for VMs. Common formats include qcow2 (copy-on-write, efficient) and raw.

Creating Disk Images

# Create a 20GB qcow2 disk image
qemu-img create -f qcow2 disk.qcow2 20G

# Create a 10GB raw disk image
qemu-img create -f raw disk.raw 10G
  • Explanation:
    • -f qcow2: Specifies the disk format (qcow2 supports snapshots and compression).
    • disk.qcow2: Output file name.
    • 20G: Disk size (supports K, M, G, T suffixes).

Inspecting Disk Images

# Display image information
qemu-img info disk.qcow2

# Check image for errors
qemu-img check disk.qcow2

Resizing Disk Images

# Increase disk size by 10GB
qemu-img resize disk.qcow2 +10G

# Shrink disk (only if unused space, requires check first)
qemu-img resize --shrink disk.qcow2 15G

Converting Disk Images

# Convert qcow2 to raw format
qemu-img convert -f qcow2 -O raw disk.qcow2 disk.raw

# Convert raw to qcow2 with compression
qemu-img convert -f raw -O qcow2 -c disk.raw disk_compressed.qcow2

Snapshots

# Create a snapshot in a qcow2 image
qemu-img snapshot -c snapshot1 disk.qcow2

# List snapshots
qemu-img snapshot -l disk.qcow2

# Revert to a snapshot
qemu-img snapshot -a snapshot1 disk.qcow2

# Delete a snapshot
qemu-img snapshot -d snapshot1 disk.qcow2

Example: Creating and Inspecting a Disk Image

Create a 30GB qcow2 image and check its details:

qemu-img create -f qcow2 ubuntu-vm.qcow2 30G
qemu-img info ubuntu-vm.qcow2
  • Output (example):
    image: ubuntu-vm.qcow2
file format: qcow2
virtual size: 30 GiB
disk size: 196 KiB

Virtual Machine Management

QEMU’s qemu-system-* commands (e.g., qemu-system-x86_64) are used to launch and manage VMs.

Basic VM Launch

Run a VM with a disk image and an ISO file:

qemu-system-x86_64 \
  -m 2G \
  -hda ubuntu-vm.qcow2 \
  -cdrom ubuntu-24.04.iso \
  -boot d \
  -enable-kvm
  • Explanation:
    • -m 2G: Allocates 2GB of RAM.
    • -hda: Specifies the primary disk image.
    • -cdrom: Attaches an ISO for installation.
    • -boot d: Boots from the CD-ROM.
    • -enable-kvm: Enables KVM for hardware acceleration.

Networking

# User-mode networking (NAT)
qemu-system-x86_64 -m 2G -hda ubuntu-vm.qcow2 -netdev user,id=net0 -device virtio-net,netdev=net0

# Bridge networking (requires setup)
qemu-system-x86_64 -m 2G -hda ubuntu-vm.qcow2 -netdev bridge,id=net0,br=br0 -device virtio-net,netdev=net0
  • Note: Bridge networking requires a configured network bridge (e.g., br0) on the host.

CPU and Memory

# Specify 4 CPU cores and 4GB RAM
qemu-system-x86_64 -m 4G -smp 4 -hda ubuntu-vm.qcow2 -enable-kvm
  • Explanation:
    • -smp 4: Allocates 4 CPU cores.
    • -m 4G: Allocates 4GB of RAM.

Example: Running a VM with VNC

Launch a VM with VNC access for remote GUI:

qemu-system-x86_64 \
  -m 2G \
  -hda ubuntu-vm.qcow2 \
  -vnc :1 \
  -enable-kvm
  • Usage: Connect to the VM using a VNC client (e.g., vncviewer localhost:5901).

Example: Installing an OS

Boot from an ISO to install Ubuntu:

qemu-system-x86_64 \
  -m 4G \
  -smp 2 \
  -hda ubuntu-vm.qcow2 \
  -cdrom ubuntu-24.04.iso \
  -boot d \
  -vnc :1 \
  -enable-kvm
  • Explanation: Boots from the ISO, allocates 4GB RAM and 2 CPUs, and provides VNC access.

Practical Script

Create a script to manage a QEMU VM and disk image:

#!/bin/bash
# manage_qemu.sh

echo "=== QEMU VM Management ==="
echo "Current directory: $(pwd)"
echo ""

# Create disk image if it doesn't exist
if [ ! -f ubuntu-vm.qcow2 ]; then
  echo "Creating disk image..."
  qemu-img create -f qcow2 ubuntu-vm.qcow2 20G
fi

echo "Disk image info:"
qemu-img info ubuntu-vm.qcow2
echo ""

echo "Starting VM..."
qemu-system-x86_64 \
  -m 2G \
  -smp 2 \
  -hda ubuntu-vm.qcow2 \
  -netdev user,id=net0 -device virtio-net,netdev=net0 \
  -enable-kvm &

echo "VM started in background. Connect via VNC (port 5900) if enabled."

Make it executable:

chmod +x manage_qemu.sh
./manage_qemu.sh

Pro Tips

Troubleshooting

  • KVM Not Working: Ensure KVM modules are loaded (lsmod | grep kvm) and your user is in the kvm group (sudo usermod -aG kvm $USER).
  • Disk Image Errors: Use qemu-img check to diagnose and repair issues.
  • VM Won’t Boot: Verify the disk image, ISO, or boot order (-boot).
  • Network Issues: Check network configuration with ip addr or ensure the correct -netdev settings.

Next Steps

In future tutorials, we’ll cover:

  • Advanced QEMU networking (e.g., TAP, bridge setups).
  • Using libvirt with QEMU for easier VM management.
  • Creating and managing VM snapshots.
  • Automating VM deployment with scripts or Ansible.

Practice Exercises

  1. Disk Management: Create a 50GB qcow2 image, resize it to 60GB, and take a snapshot.
  2. VM Setup: Launch a VM with 4GB RAM, 2 CPUs, and user-mode networking.
  3. OS Installation: Install a Linux distro using an ISO and configure VNC access.
  4. Automation: Write a script to start multiple VMs with different disk images.

Resources

Practice QEMU commands to master disk image and VM management!