KVM - Fast ways to spin up VMs - Cloud Init

I'm using plain KVM + Libvirt as my hypervisor of choice in my Homelab since it gives me a lot of flexibility, reliability and performance. Installing VMs using traditional installers allows for customizations during install but if all you're doing is quickly spinning up a VM to test something, pre-built Cloud Images are probably a better choice. 

The Cloud Images can be customized though before importing them using tools like virt-sysprep or cloud-init.

In this article, I'll be covering a workflow using provided Cloud Images and Cloud Init to bootstrap ephemeral Linux Servers.

First, we'll have to download the cloud image, I'll be using a Amazonlinux Cloud Image this time:

 [root@hyv02 ~]# curl -4 -f -k -L -Z -o '/var/kvm/nfs-vm-templates/amazonlinux-2023-2025-07-21-x86_64.qcow2' -X 'GET' -H 'Accept: application/octet-stream' -H 'User-Agent: curl/1.33.7' https://cdn.amazonlinux.com/al2023/os-images/2023.8.20250721.2/kvm/al2023-kvm-2023.8.20250721.2-kernel-6.1-x86_64.xfs.gpt.qcow2  
 [root@hyv02 ~]# chown root:root /var/kvm/nfs-vm-templates/amazonlinux-2023-2025-07-21-x86_64.qcow2; chmod 600 /var/kvm/nfs-vm-templates/amazonlinux-2023-2025-07-21-x86_64.qcow2  

You can now clone the cloud-image and create a new qcow2 image using it as a reference for the new VM:

 [root@hyv02 ~]# qemu-img create -b /var/kvm/nfs-vm-templates/amazonlinux-2023-2025-07-21-x86_64.qcow2 -f qcow2 -F qcow2 /var/kvm/nfs-vm-data/amazonlinux-cloud-init.qcow2 10G  

I'll not create a few Files required for Cloud Init and the Cloud Init Data ISO File.

 [root@hyv02 ~]# mkdir -p -m 700 ~/cloud-init  
 [root@hyv02 ~]# cat << EOF > ~/cloud-init/meta-data  
 instance-id: almalinux-cloud-init  
 local-hostname: amazonlinux-cloud-init  
 EOF  
 [root@hyv02 ~]# cat << EOF > ~/cloud-init/user-data  
 #cloud-config  
 create_hostname_file: true  
 preserve_hostname: false  
 hostname: almalinux-cloud-init  
 users:  
   - name: archy  
     groups: sudo  
     shell: /bin/bash  
     sudo: ['ALL=(ALL:ALL) NOPASSWD:ALL']  
     ssh_authorized_keys:  
       - ssh-ed25519 AAAAC3Nza1  
       - ssh-ed25519 AAAAC3Nza2  
 EOF  
 [root@hyv02 ~]# cat << EOF > ~/cloud-init/network-config  
 version: 2  
 ethernets:  
   id0:  
     match:  
       name: en*  
     dhcp4: true  
     dhcp6: false  
 EOF  
 [root@hyv02 ~]# genisoimage -output /var/kvm/nfs-vm-templates/cidata.iso -V cidata -r -J ~/cloud-init/user-data ~/cloud-init/meta-data ~/cloud-init/network-config  

Refresh all storage pools for good measure:

 [root@hyv02 ~]# for pool in $(virsh -c 'qemu:///system' pool-list --name | awk NF); do virsh -c 'qemu:///system' pool-refresh --pool ${pool}; done  

From here on out you can use virt-install, virt-manager or cockpit machines to create a vm and start the VM. An example for virt-install could look like this:

 [root@hyv02 ~]# virt-install --import --hvm --noautoconsole \  
     --connect 'qemu:///system' \  
     --name 'amazonlinux-cloud-init' \  
     --memory '1024' \  
     --vcpus '2' \  
     --cpu 'host-model' \  
     --controller 'type=virtio-serial' \  
     --disk '/var/kvm/nfs-vm-data/amazonlinux-cloud-init.qcow2,bus=virtio' \  
     --disk '/var/kvm/nfs-vm-templates/cidata.iso,device=cdrom' \  
     --network 'bridge=br-internal,model=virtio' \  
     --arch 'x86_64' \  
     --machine 'q35' \  
     --os-variant 'linux2024' \  
     --rng '/dev/urandom'  

This command will create a VM named 'amazonlinux-cloud-init' with 1024MB of RAM and 2 CPU Cores, utilizing the existing Disk and adding the Cloud Init Data ISO created earlier.

Feel free to comment and / or suggest a topic.