Cloud Image Usage

Enterprise Linux distributions (AlmaLinux, Rocky Linux) provide pre-built cloud images distributed in qcow2 format. These images are small in size, boot quickly, come with Cloud-Init pre-installed, and are well-suited for rapid VM deployment on KVM/libvirt, OpenStack, or other virtualization platforms.

Downloading Official Cloud Images

AlmaLinux Cloud Images

# AlmaLinux 10 / 9 / 8 GenericCloud images (just change the version; the 10 path returns 200)
wget https://repo.almalinux.org/almalinux/10/cloud/x86_64/images/AlmaLinux-10-GenericCloud-latest.x86_64.qcow2
wget https://repo.almalinux.org/almalinux/9/cloud/x86_64/images/AlmaLinux-9-GenericCloud-latest.x86_64.qcow2

# AlmaLinux 8 GenericCloud image
wget https://repo.almalinux.org/almalinux/8/cloud/x86_64/images/AlmaLinux-8-GenericCloud-latest.x86_64.qcow2

# Verify download integrity
wget https://repo.almalinux.org/almalinux/9/cloud/x86_64/images/CHECKSUM
sha256sum -c CHECKSUM --ignore-missing

Rocky Linux Cloud Images

# Rocky Linux 9 GenericCloud image
wget https://download.rockylinux.org/pub/rocky/9/images/x86_64/Rocky-9-GenericCloud.latest.x86_64.qcow2

# Rocky Linux 8 GenericCloud image
wget https://download.rockylinux.org/pub/rocky/8/images/x86_64/Rocky-8-GenericCloud.latest.x86_64.qcow2

# Verify checksums
wget https://download.rockylinux.org/pub/rocky/9/images/x86_64/CHECKSUM
sha256sum -c CHECKSUM --ignore-missing

Viewing Image Information

# Install the qemu-img tool
sudo dnf install -y qemu-img

# View image information
qemu-img info AlmaLinux-9-GenericCloud-latest.x86_64.qcow2

Example output:

image: AlmaLinux-9-GenericCloud-latest.x86_64.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 582 MiB
cluster_size: 65536
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false

Customizing Images with virt-customize

virt-customize is a tool provided by libguestfs-tools that can modify disk images without booting a virtual machine.

Installing the Tools

sudo dnf install -y libguestfs-tools

Basic Customization Operations

# Make a copy of the image for customization (preserve the original)
cp AlmaLinux-9-GenericCloud-latest.x86_64.qcow2 almalinux9-custom.qcow2

# Set root password
sudo virt-customize -a almalinux9-custom.qcow2 \
  --root-password password:YourSecurePassword123

# Install packages
sudo virt-customize -a almalinux9-custom.qcow2 \
  --install vim-enhanced,htop,tmux,git,wget,curl

# Enable services
sudo virt-customize -a almalinux9-custom.qcow2 \
  --run-command 'systemctl enable firewalld'

# Write files
sudo virt-customize -a almalinux9-custom.qcow2 \
  --write '/etc/motd:Welcome to Custom AlmaLinux 9 Image'

# Upload a local file into the image
sudo virt-customize -a almalinux9-custom.qcow2 \
  --upload /path/to/local/sshd_config:/etc/ssh/sshd_config

# Set hostname
sudo virt-customize -a almalinux9-custom.qcow2 \
  --hostname custom-server

# Set timezone
sudo virt-customize -a almalinux9-custom.qcow2 \
  --timezone Asia/Shanghai

# Run custom scripts
sudo virt-customize -a almalinux9-custom.qcow2 \
  --run-command 'echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.d/99-custom.conf'

# Set SELinux relabel (recommended after modifying files)
sudo virt-customize -a almalinux9-custom.qcow2 \
  --selinux-relabel

Complete Image Customization Workflow

#!/bin/bash
# customize-image.sh - Batch customize cloud images

SOURCE_IMAGE="AlmaLinux-9-GenericCloud-latest.x86_64.qcow2"
CUSTOM_IMAGE="almalinux9-production.qcow2"

# Copy the original image
cp "${SOURCE_IMAGE}" "${CUSTOM_IMAGE}"

# Expand disk to 30GB
qemu-img resize "${CUSTOM_IMAGE}" 30G

# Perform customization
sudo virt-customize -a "${CUSTOM_IMAGE}" \
  --root-password password:TempPassword123 \
  --timezone Asia/Shanghai \
  --install epel-release \
  --install vim-enhanced,htop,tmux,git,wget,curl,net-tools,bind-utils \
  --install firewalld,fail2ban,chrony \
  --run-command 'systemctl enable firewalld' \
  --run-command 'systemctl enable fail2ban' \
  --run-command 'systemctl enable chronyd' \
  --write '/etc/sysctl.d/99-custom.conf:net.core.somaxconn = 65535
net.ipv4.tcp_max_syn_backlog = 65535
vm.swappiness = 10' \
  --run-command 'echo "PermitRootLogin prohibit-password" >> /etc/ssh/sshd_config.d/99-custom.conf' \
  --selinux-relabel

echo "Image customization complete: ${CUSTOM_IMAGE}"
qemu-img info "${CUSTOM_IMAGE}"

Cleaning Images with virt-sysprep

Before distributing a custom image, you should clean out instance-specific information:

# Clean the image (removes SSH keys, machine IDs, logs, etc.)
sudo virt-sysprep -a almalinux9-custom.qcow2

# Specify which cleanup operations to perform
sudo virt-sysprep -a almalinux9-custom.qcow2 \
  --operations defaults,-ssh-userdir

# List all available cleanup operations
sudo virt-sysprep --list-operations

Creating Cloud Instances with virt-install

Installing Virtualization Tools

# Install KVM and management tools
sudo dnf install -y @virtualization
sudo systemctl enable --now libvirtd

# Verify virtualization support
virt-host-validate

Preparing Cloud-Init Configuration

# Create a working directory
mkdir -p /var/lib/libvirt/cloud-init

# Create meta-data
cat > /var/lib/libvirt/cloud-init/meta-data << 'EOF'
instance-id: vm-001
local-hostname: web-server-01
EOF

# Create user-data
cat > /var/lib/libvirt/cloud-init/user-data << 'EOF'
#cloud-config

hostname: web-server-01
fqdn: web-server-01.example.com

users:
  - default
  - name: admin
    groups: wheel
    sudo: ALL=(ALL) NOPASSWD:ALL
    shell: /bin/bash
    ssh_authorized_keys:
      - ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAI... admin@workstation

package_update: true
packages:
  - vim-enhanced
  - htop
  - nginx

runcmd:
  - systemctl enable --now nginx
EOF

# Create network-config (optional, for static IP configuration)
cat > /var/lib/libvirt/cloud-init/network-config << 'EOF'
version: 2
ethernets:
  eth0:
    dhcp4: true
EOF

Generating the Cloud-Init ISO

# Install genisoimage
sudo dnf install -y genisoimage

# Generate the seed ISO
genisoimage -output /var/lib/libvirt/cloud-init/seed.iso \
  -volid cidata \
  -joliet -rock \
  /var/lib/libvirt/cloud-init/user-data \
  /var/lib/libvirt/cloud-init/meta-data \
  /var/lib/libvirt/cloud-init/network-config

Creating a Virtual Machine

# Prepare the disk image (copy cloud image as the VM disk)
VM_NAME="web-server-01"
IMAGE_DIR="/var/lib/libvirt/images"

sudo cp almalinux9-custom.qcow2 "${IMAGE_DIR}/${VM_NAME}.qcow2"

# Resize the disk
sudo qemu-img resize "${IMAGE_DIR}/${VM_NAME}.qcow2" 30G

# Create the VM with virt-install
sudo virt-install \
  --name "${VM_NAME}" \
  --memory 2048 \
  --vcpus 2 \
  --disk "${IMAGE_DIR}/${VM_NAME}.qcow2",format=qcow2 \
  --disk /var/lib/libvirt/cloud-init/seed.iso,device=cdrom \
  --os-variant almalinux9 \
  --network bridge=virbr0,model=virtio \
  --graphics none \
  --console pty,target_type=serial \
  --import \
  --noautoconsole

# Check VM status
sudo virsh list --all

# View VM IP address
sudo virsh domifaddr "${VM_NAME}"

Batch Creating Virtual Machines

#!/bin/bash
# create-vms.sh - Batch create virtual machines

BASE_IMAGE="/var/lib/libvirt/images/almalinux9-custom.qcow2"
IMAGE_DIR="/var/lib/libvirt/images"
CI_DIR="/var/lib/libvirt/cloud-init"
SSH_KEY="ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAI... admin@workstation"

VMS=("web-01" "web-02" "db-01")
MEMORY=2048
VCPUS=2
DISK_SIZE="30G"

for VM_NAME in "${VMS[@]}"; do
  echo "=== Creating VM: ${VM_NAME} ==="

  # Prepare the disk
  sudo cp "${BASE_IMAGE}" "${IMAGE_DIR}/${VM_NAME}.qcow2"
  sudo qemu-img resize "${IMAGE_DIR}/${VM_NAME}.qcow2" "${DISK_SIZE}"

  # Generate cloud-init configuration
  WORK_DIR="${CI_DIR}/${VM_NAME}"
  mkdir -p "${WORK_DIR}"

  cat > "${WORK_DIR}/meta-data" << METAEOF
instance-id: ${VM_NAME}
local-hostname: ${VM_NAME}
METAEOF

  cat > "${WORK_DIR}/user-data" << USEREOF
#cloud-config
hostname: ${VM_NAME}
users:
  - default
  - name: admin
    groups: wheel
    sudo: ALL=(ALL) NOPASSWD:ALL
    shell: /bin/bash
    ssh_authorized_keys:
      - ${SSH_KEY}
package_update: true
packages:
  - vim-enhanced
  - htop
USEREOF

  # Generate ISO
  genisoimage -output "${WORK_DIR}/seed.iso" \
    -volid cidata -joliet -rock \
    "${WORK_DIR}/user-data" \
    "${WORK_DIR}/meta-data"

  # Create the VM
  sudo virt-install \
    --name "${VM_NAME}" \
    --memory ${MEMORY} \
    --vcpus ${VCPUS} \
    --disk "${IMAGE_DIR}/${VM_NAME}.qcow2",format=qcow2 \
    --disk "${WORK_DIR}/seed.iso",device=cdrom \
    --os-variant almalinux9 \
    --network bridge=virbr0,model=virtio \
    --graphics none \
    --console pty,target_type=serial \
    --import \
    --noautoconsole

  echo "${VM_NAME} created successfully"
done

echo ""
echo "=== All VM Status ==="
sudo virsh list --all

Resizing Images

Expanding Disk Capacity

# View current size
qemu-img info /var/lib/libvirt/images/web-server-01.qcow2

# Expand to 50GB (VM must be shut down)
sudo virsh shutdown web-server-01
sudo qemu-img resize /var/lib/libvirt/images/web-server-01.qcow2 50G
sudo virsh start web-server-01

After the VM boots, you still need to expand the partition and filesystem:

# Execute the following after logging into the VM
# View disk layout
lsblk

# If using LVM (typical for cloud images)
sudo growpart /dev/vda 4        # Expand the partition (partition number may vary)
sudo pvresize /dev/vda4         # Expand the PV
sudo lvextend -l +100%FREE /dev/mapper/almalinux-root  # Expand the LV
sudo xfs_growfs /               # Expand the XFS filesystem
# Or: sudo resize2fs /dev/mapper/almalinux-root  # For ext4

# If not using LVM (simple partitioning)
sudo growpart /dev/vda 1
sudo xfs_growfs /

Compressing Images

# Shut down the VM
sudo virsh shutdown web-server-01

# Compress the qcow2 image (remove sparse space)
sudo qemu-img convert -O qcow2 -c \
  /var/lib/libvirt/images/web-server-01.qcow2 \
  /var/lib/libvirt/images/web-server-01-compressed.qcow2

# Use virt-sparsify for more thorough compression
sudo virt-sparsify --in-place /var/lib/libvirt/images/web-server-01.qcow2

Format Conversion

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

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

# qcow2 to vmdk (VMware format)
qemu-img convert -f qcow2 -O vmdk image.qcow2 image.vmdk

# qcow2 to vdi (VirtualBox format)
qemu-img convert -f qcow2 -O vdi image.qcow2 image.vdi

Common VM Management Commands

# List all virtual machines
sudo virsh list --all

# Start/Shutdown/Reboot
sudo virsh start web-server-01
sudo virsh shutdown web-server-01
sudo virsh reboot web-server-01

# Force shutdown
sudo virsh destroy web-server-01

# Connect to the VM console
sudo virsh console web-server-01
# Exit console: Ctrl+]

# View VM information
sudo virsh dominfo web-server-01
sudo virsh domifaddr web-server-01

# Create a snapshot
sudo virsh snapshot-create-as web-server-01 \
  --name "before-upgrade" \
  --description "Snapshot before upgrade"

# List snapshots
sudo virsh snapshot-list web-server-01

# Revert to a snapshot
sudo virsh snapshot-revert web-server-01 before-upgrade

# Delete a VM (keep disks)
sudo virsh undefine web-server-01

# Delete a VM (remove disks as well)
sudo virsh undefine web-server-01 --remove-all-storage

Using official cloud images with Cloud-Init and virt-install allows you to rapidly create standardized VM instances in a local KVM environment. This experience is very similar to creating ECS instances on a public cloud, making it an ideal solution for building private clouds and development/testing environments.