KVM, QEMU, and Libvirt Management

Create a VM from an ISO Image

To install a new virtual machine from an ISO file, use the virt-install command.

Key virt-install Parameters:

Parameter	Description
`--name`	The unique name of the VM.
`--os-type`	Optimizes configuration for an OS type (e.g., 'linux', 'windows').
`--os-variant`	Fine-tuned optimization for specific versions (e.g., 'fedora8', 'win10').
`--ram`	Memory allocated to the guest in MiB.
`--vcpus`	Number of virtual CPUs to configure.
`--disk path=...`	Storage configuration and path to the media.
`--graphics`	Specifies the graphical display (e.g., 'spice', 'vnc' or 'none').
`--location`	Source for the installation (ISO, URL, or local directory).
`--extra-args`	Additional kernel command-line arguments for the installer.

Disk Image Management

Advanced QEMU-IMG Commands

Check the file type and details:

sudo file /var/lib/libvirt/images/debian9.img
# Output: QEMU QCOW Image (v3), 10737418240 bytes

Convert a disk image format (e.g., raw to qcow2):

qemu-img convert -f raw -O qcow2 image.raw image.qcow2

Resize and verify:

qemu-img resize bionic-server-cloudimg-amd64.img 20G
qemu-img info bionic-server-cloudimg-amd64.img

Manual QEMU Execution

You can start a VM directly without libvirt for testing purposes:

qemu-system-x86_64 -m 1024 -hda debian.qcow2 -cdrom debian-inst.iso -boot d

Configure a Pre-defined VM Image

Download and customize official cloud images using virt-customize:

wget [https://cloud-images.ubuntu.com/bionic/current/bionic-server-cloudimg-amd64.img](https://cloud-images.ubuntu.com/bionic/current/bionic-server-cloudimg-amd64.img)

# Set root password and remove cloud-init
sudo virt-customize -a bionic-server-cloudimg-amd64.img --root-password password:coolpass
sudo virt-customize -a bionic-server-cloudimg-amd64.img --uninstall cloud-init

VM Life-cycle and Snapshots

Define and Start

virsh define vm.xml
virsh start bionic-vm

Snapshot Management

Libvirt allows taking snapshots of a running or stopped VM (requires qcow2):

# Create a snapshot
virsh snapshot-create-as --domain bionic-vm --name "AfterConfig" --description "Initial Setup"

# List snapshots
virsh snapshot-list bionic-vm

# Restore a snapshot
virsh snapshot-revert bionic-vm --snapshotname "AfterConfig"

# Delete a snapshot
virsh snapshot-delete bionic-vm --snapshotname "AfterConfig"

Console Access

virsh console bionic-vm

To exit the console, use Ctrl + Alt Gr + ] (or Ctrl + ]).

Networking

Virtual Networks

virsh net-list --all

Default NAT bridge (virbr0) configuration:

<network>
  <name>default</name>
  <forward mode='nat'/>
  <bridge name='virbr0' stp='on' delay='0'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.2' end='192.168.122.254'/>
    </dhcp>
  </ip>
</network>

Guest Network (Netplan)

Configure /etc/netplan/01-netcfg.yaml:

network:
  version: 2
  renderer: networkd
  ethernets:
    ens2:
      dhcp4: true

Apply: sudo netplan --debug apply

Performance and Resource Tuning

CPU Pinning and Memory

View VM resource usage:

virsh domstats bionic-vm

Set memory limits on the fly:

virsh setmem bionic-vm 2048M --config --live

Troubleshooting: Shutdown Issues

If virsh shutdown fails:

ACPI Features: Ensure the XML has ACPI enabled:

<features>
  <acpi/>
  <apic/>
</features>

Guest Agent: Install qemu-guest-agent inside the VM.
Emergency Stop: virsh destroy bionic-vm