How to create a KVM template on Arch Linux

Posted on Fri 30 October 2020 in arch

If you are deploying many virtual machines, templates can be leveraged to save time and even storage if using thin provisioning.

The process of creating a template is two steps:

  1. Create and configure the base operating system image
  2. Use sysprep to remove system-specific settings

Once completed we can create virtual machines based on the templates.

Install libraries and enable KVM services

pacman -S virt-manager qemu vde2 iptables ebtables dnsmasq bridge-utils openbsd-netcat libguestfs cpio

# enable services
systemctl enable libvirtd.service
systemctl start libvirtd.service

Allow user account to use KVM

Edit /etc/libvirt/libvirtd.conf

# uncomment the following
unix_sock_group = "libvirt"
# be careful to uncomment the 'rw' permissions not the 'ro'
unix_sock_rw_perms = "0770"

Add current user to libvirt group

usermod -a -G libvirt $(whoami)
newgrp libvirt 
# the newgrp command is a gem, no need to logout for the group to take effect

Restart the libvirt daemon:

systemctl restart libvirtd.service

Create image for base Ubuntu install

qemu-img create -o preallocation=metadata -f qcow2 images/Template-ubuntu20.04.qcow2 10G

Start the default network

virsh net-start default
virsh net-autostart default

Install the guest operating system

virt-install --virt-type kvm --name Template-ubuntu20.04 --ram 8196  --disk /usr/libvirt/images/ubuntu.qcow2,format=qcow2 
   --vcpus 4 
   --network network=default 
   --graphics vnc,listen= --noautoconsole 
   --os-type=linux --os-variant=ubuntu20.04 

From the host, SSH into the guest to verify it is working

You can use the following command to find the ip address of the guest machine:

virsh net-dhcp-leases default | awk '{print $5, $6}'

Use the ip address above to test SSH is working

Configure the guest operating system

Continue to configure the system. At the very least update the system:

sudo apt-get update
sudo apt-get upgrade

Sysprep the image to prepare for template

virt-sysprep -v -d Template-ubuntu20.04
virsh undefine Template-ubuntu20.04

Dump the configuration to XML

virsh dumpxml ubuntu20.04-base > /usr/libvirt/virtual_machines/images/base-image.xml

Create a clone using the template

# first create a new image using the base template
qemu-img create -b Template-ubuntu20.04.qcow2 -F qcow2 -f qcow2 ubuntu-clone.qcow2

# resize the image to be an appropriate size
qemu-img resize /usr/libvirt/virtual_machines/images/ubuntu-clone.qcow2 +50G

# clone the virtual machine,
virt-clone --original-xml=/usr/libvirt/virtual_machines/images/base-image.xml -f /usr/libvirt/virtual_machines/images/ubuntuXX.qcow2 -n ubuntuXX --preserve-data

Your virtual machine is now cloned using the thin provisioning method.