KVM - Fast ways to spin up VMs

KVM - Fast ways to spin up VMs - Virt-Sysprep

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 my workflow using virt-sysprep with a Alma Cloud Image although any other cloud image should work.

 [root@hyv02 ~]# curl -4 -f -k -L -Z -o '/var/kvm/nfs-vm-templates/almalinux-9-2025-05-22-x86_64.qcow2' -X 'GET' -H 'Accept: application/octet-stream' https://raw.repo.almalinux.org/almalinux/9/cloud/x86_64/images/AlmaLinux-9-GenericCloud-9.6-20250522.x86_64.qcow2  
 [root@hyv02 ~]# chown root:root /var/kvm/nfs-vm-templates/almalinux-9-2025-05-22.x86_64.qcow2; chmod 600 /var/kvm/nfs-vm-templates/almalinux-9-2025-05-22.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/almalinux-9-2025-05-22-x86_64.qcow2 -f qcow2 -F qcow2 /var/kvm/nfs-vm-data/almalinux-virt-sysprep.qcow2 10G

This will now create a new Image using the Cloud Image as basis for the ephemeral testing vm.

Now, let's run virt-sysprep to do the following tasks:

set the hostname
set the timezone
set a root password
create a user and group with custom uid/gid
modify sudo to allow passwordless sudo for the newly created user
since we're using a SELinux system, disable SELinux temporarily for firstboot to run
set a firstboot command to enable selinux, relabel the whole system and reboot
also inject a few SSH Keys that I would like to add to my user's authorized_keys

 [root@hyv02 ~]# virt-sysprep -c 'qemu:///system' --format 'qcow2' --add '/var/kvm/nfs-vm-data/almalinux-virt-sysprep-disk1.qcow2' \  
     --hostname 'almalinux-virt-sysprep' --timezone 'Europe/Prague' --root-password 'password:provisioned-by-virt-sysprep' --password-crypto 'sha512' \  
     --run-command 'groupadd -g "10117" archy' \  
     --run-command 'useradd -m -c "privisioned by virt-sysprep" -d "/home/archy" -g "archy" -s "/bin/bash" -u "10117" archy" \  
     --run-command 'echo -e "archy ALL=(ALL:ALL) NOPASSWD:ALL" > /etc/sudoers.d/archy' \  
     --run-command 'sed -i /etc/selinux/config -E -e "s/^SELINUX=.*/SELINUX=permissive/g"' \  
     --firstboot-command 'sed -i /etc/selinux/config -E -e "s/^SELINUX=.*/SELINUX=enforcing/g"; touch /.autorelabel; systemctl reboot' \  
     --ssh-inject 'archy:string:ssh-ed25519 AAAAC3Nza1...' \  
     --ssh-inject 'archy:string:ssh-ed25519 AAAAC3Nza2...' \  
     --ssh-inject 'archy:string:ssh-ed25519 AAAAC3Nza3...' \  
     --selinux-relabel --colors --no-network

The flags '--network|--no-network' tell virt-sysprep to either enable or disable network connections during sysprepping.

After the command finished, the image is now ready to be imported using tools like virt-install, virt-manager or cockpit-machines.

An Example of installing the VM using virt-install could look like this:

 [root@hyv02 ~]# virt-install --import --hvm --noautoconsole \  
   --connect 'qemu:///system' \  
   --name 'almalinux-virt-sysprep' \  
   --memory '1024' \  
   --vcpus '2' \  
   --cpu 'host-model' \  
   --controller 'type=virtio-serial' \  
   --disk '/var/kvm/nfs-vm-data/almalinux-virt-sysprep-disk1.qcow2,bus=virtio' \  
   --network 'bridge=br-internal,model=virtio' \  
   --arch 'x86_64' \  
   --machine 'q35' \  
   --os-variant 'almalinux9' \  
   --rng '/dev/urandom'

This will create a vm named 'almalinux-virt-sysprep' using the disk we just prepared with 1024MB of RAM and 2 CPU Cores. The disk will be attached as VirtIO Device.

Feel free t commend and / or suggest a topic.

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