As we have seen a little into KVM/QEMU, Libvirt in the previous post, this post will demonstrate how to create, manage and remove virtual machines.

Note

I use Void Linux as my base operating system (recently hoped from Manjaro), so the commands will be according to it. But most of the packages are some with similar or different names.

Before installing packages for KVM/QEMU, we need to check for KVM support.

Prerequisites for KVM:

  • To use KVM the host’s processor should support virtualization. It is named different for different processor VT-x for Intel and AMD-V for AMD processors.

  • To check whether the system provides the support for virtualization for either CPUs, use the following command:

LC_ALL=C lscpu | grep Virtualization

Or

grep -E 'vmx|svm|0xc0f' /proc/cpuinfo

If nothing is displayed after running either of the two commands, then the processor does not support hardware virtualization, and will unable to use KVM.

The virtualization support can be turned on in the BIOS (if available).

  • To enable KVM, your user must be added to the KVM group:
usermod -aG kvm <username>
  • To check whether the modules are loaded in the kernel, use the following command:
lsmod | grep kvm

If the output is either kvm_intel or kvm_amd, the modules are loaded.

If the modules are not loaded, use the following command to do so:

modprobe kvm-amd # for AMD CPUs

modprobe kvm-intel # for Intel CPUs
  • Kernel version should be greater than 2.6.60.

Packages and Dependencies

If everything is set right, we need to install the packages and dependencies that are required to create/manage a VM.

  • qemu - This package provides the x86_64 architecture emulators for both full-system emulation (qemu-system-x86_64) and usermode (qemu-x86_64) variants.
xbps-install -S qemu
  • bridge-utils - Bridges the physical interfaces and the VM interfaces
xbps-install -S bridge-utils
  • libvirt - The libvirt package provides the daemon libvirtd which handles library calls, manages virtual machines and hypervisor controls.
xbps-install -S libvirt

Add the user to the libvirt group:

usermod -aG libvirt <user>

To start the libvirtd daemon, run:

ln -s /etc/sv/libvirtd /var/service

To check the status:

sv status libvirtd
  • virt-manager - virt-manager is a graphical tool for managing virtual machines via libvirt. It includes virt-install, virt-clone, virt-xml.
xbps-install -S virt-manager

Start the virtlogd service:

ln -s /etc/sv/virtlogd/ /var/service/
  • libguestfs - libguestfs is a set of tools for accessing and modifying virtual machine (VM) disk images. It can access alomost any disk image including Vmware’s VMDK & Hyper-V disk format.
xbps-install -S libguestfs

This package includes virsh.

  • virt-viewer - Virt Viewer provides a graphical viewer for the guest OS display. At this time is supports guest OS using the VNC or SPICE protocols.
xbps-install -S virt-viewer

To install all the packages

xbps-install -S qemu bridge-utils libvirt virt-manager libguestfs virt-viewer

Creating a VM

Now that we have installed all the required packages and its dependencies, we can now proceed to create a virtual machine with virt-install.

Now before getting into it, the following are the details of the VM to be created:

VM Name   -   Manjaro
NETWORK   -   virbr0 (virbr0/br0)
RAM       -   4096
CPU       -   4
DISK      -   25GB
CD-ROM    -   /vol1/ISO/manjaro.iso
  • virt-install options that are required to create a VM:
Options Values Description
--connect qemu:///system Connect to the localhost KVM
--name Manjaro Name of the VM
--ram 4096 Size of RAM in MB
--vcpus 4 No. of vcpus
--os-type linux OS type
--os-variant manjaro OS variant (use osinfo-query os)
--network network=virbr0 Bridge for network connectivity
--graphics spice Configure guest display settings
--virt-type kvm Virtualization type as KVM or Xen
--disk size 25 Disk Space of the VM
--cdrom /path/of/iso Path for the ISO file 
virt-install --connect qemu:///system --name Manjaro --ram 4096 --vcpus 4 --os-type linux --os-variant manjaro --network bridge=virbr0 --graphics spice --virt-type kvm --disk size=25 --cdrom /vol1/ISO/manjaro-xfce-20.0.1-200511-linux56.iso
  • To shutdown the machine
virsh shutdown <domain-name>

Managing virtual machines with virsh

  • The user can create, manage, delete, start and stop virtual machines with the virsh command.

  • It can be used in scripts for automating some of the aspects of managing the virtual machine.

  • The terminology virsh uses is a little different. The term domain will be used instead of the term guest virtual machine.

  • The virtual machine that is created in one user cannot be retrieved by other user. To access all the virtual machines in a system, change the livbirt URL to access all the guests in a system.

virsh --connect qemu:///system list --all

The above command outputs the guest machines in a single system including all the ones from different users.

  • The domain name of the machine create above is Manjaro.

  • To start a virtual machine

virsh --connect qemu:///system start Manjaro
  • To reboot a virtual machine
virsh --connect qemu:///system reboot Manjaro
  • To shutdown a virtual machine
virsh --connect qemu:///system shutdown Manjaro
  • To suspend a virtual machine
virsh --connect qemu:///system suspend Manjaro
  • To resume a virtual machine
virsh --connect qemu:///system resume Manjaro

To edit the storage path of the virtual machine

In KVM, the virtual machines are store in /var/lib/libvirt/images. The default storage path can be changed to the users preference.

  • First, login to the host machine. Shutdown all the VMs, if any loaded.

  • To list the storage pool

virsh --connect qemu:///system pool-list
  • To get the info about the storage pool
virsh --connect qemu:///system pool-info default
  • To get the path of the storage pool
virsh --connect qemu:///system pool-dumpxml default | grep -i path
  • To check whether the existing VMs are located in the pool path
virsh --connect qemu:///system vol-list default | grep "/var/lib/libvirt/images"

virsh --connect qemu:///system vol-list default
  • Before changing the path, stop the storage pool
virsh --connect qemu:///system pool-destroy default
  • Edit the default pool configuration
virsh --connect qemu:///system pool-edit default
  • Edit the default pool configuration
virsh --connect qemu:///system pool-edit default

  • Change the path in the <path>your_new_path</path>

  • After editing the path, start the storage pool.

virsh --connect qemu:///system pool-start default
  • To verify
virsh --connect qemu:///system pool-dumpxml default | grep -i path
  • To move the exciting VMs to the new path
mv /var/lib/libvirt/images/<name_of_vm>.qcow2 /your/new/path/
  • Edit the VM config’s to the new path
virsh --connect qemu:///system edit Manjaro

Change the source file='/var/lib/libvirt/images/<name_of_vm>.qcow2' to source file='\your\new\path\<name_of_vm>.qcow2'

  • Everything is set. Start the KVM guest.
virsh --connect qemu:///system start Manjaro

Add virtual disk to an active virtual machine

To add extra disk space to an existing VM

  • qemu-img is used to create a virtual disk
qemu-img resize Manjaro.qcow2 +10G

This add (‘+’) 10GB to the virtual machine.

Add Memory to the virtual machine

For example, lets try to increase the memory of the virtual machine from 4GB to 6GB.

  • Shutdown the virtual machine

  • Using virsh edit edit the VM

virsh --connect qemu:///system edit Manjaro

  • Change the value of the memory size (in KiB) in <memory unit='KiB'>4194304</memory> to <memory unit='KiB'>6291456</memory>

  • Save the file and exit. Restarting the VM will show the updated configuration.

Add VCPUs to a virtual machine

To increase the number of VCPUs allocated to a virtual machine edit the xml file.

  • Shutdown the virtual machine.

  • Using virsh edit edit the configuration of the virtual machine.

  • Change the value from <vcpu placement='static'>2</vcpu> to <vcpu placement='static'>4</vcpu>

  • Save and exit. Restarting the virtual machine should change the configuration.

Saving the configuration and recreating

The configuration of virtual machine can be saved and can be used to recreate the virtual machine.

  • To Save
virsh --connect qemu:///connect dumpxml Manjaro > Manjaro.xml
  • To recreate the machine from the xml configuration
virsh --connect qemu:///system create Manjaro.xml

Delete the virtual machine

  • Shutdown the virtual machine

  • Destroy the virtual machine

virsh --connect qemu:///system destroy Manjaro
  • Undefine the virtual machine
virsh --connect qemu:///system undefine Manjaro
  • Now, remove the disk image file from the system
rm /var/lib/libvirt/images/Manjaro.qcow2