Libvirt and KVM

Mon 25 June 2012 by Thejaswi Puthraya

Cross-posted from my work blog.

Libvirt and KVM

"But it works on my local setup!" We have heard or probably said this tens of times after something that we deployed to the production server breaks. After fire fighting for hours we learn that a particular package's version varies from the local setup. This is a fairly common problem that plagues every developer. Off late, quite a lot of interest and work is going into reducing the dev/prod parity to prevent such problems.

Software like virtualenv help a great deal in compartmentalizing python dependencies but most web applications nowadays have to deal with loads of other dependencies like web servers, application servers etc.

We use Ubuntu at work (not a rigid requirement) and they have biannual releases with long term releases LTS every two years. We use LTS releases for our servers and the general releases as our development desktops.

In such a case, how do we reduce the parity with our server setup? This was already explained in a blog post earlier by Dheeraj. While Vagrant is a fine piece of software, Virtualbox on Linux based operating systems tends to be very problematic. There are a lot of random crashes, kernel dumps, disk corruptions etc and you spend a lot of time repairing it rather than concentrating on developing. This is where libvirt and KVM come in.

Libvirt is a common API that helps manage virtualization platforms (or hypervisors in it's terminology). Through this single API, you will be able to talk to a host of hypervisors like Virtualbox, KVM, Xen, LXC, OpenVZ, VMware based hypervisors and also Microsoft Hyper-V. Libvirt also provides for network and disk management, authentication and access control etc.

KVM (Kernel Virtual Machine) is a Linux kernel module (requires no compilation like Xen) that supports native (hardware) virtualization. KVM based guests almost perform as well as the host and since it is part of the mainline kernel since 2.6.20, you may consider it the 'official' virtualization platform on most Linux based distros.

Qemu is a userspace program that talks to the KVM module and emulates some hardware devices.

A guest (or client/domain as some may refer) is a virtual machine running on top of a host (or server).

So far we have just dealt with the definitions, let's install something to play around with. But before that, here's the customary disclaimer.


KVM works only on Linux based distributions and virtualization ready hardware.

Installing software on ubuntu is fairly easy thanks to apt-get.

  • Let's install libvirt and qemu-kvm (the userspace program mentioned above) first:

    $ sudo apt-get install libvirt-bin qemu-kvm
  • You will have to add the user who can access libvirt to the libvirtd group:

    $ sudo adduser <username> libvirtd

    If you have added the current user (specified by <username>), log out and login to have the groups addition to get refreshed:

    $ groups <username>
    <username> : <username> libvirtd
  • After you have successfully installed libvirt, check if virsh (virtualization shell) works fine:

    $ virsh
    virsh #
  • Now, you may create your guest VM in either of the two ways specified below, through the CLI tool (virsh) or the graphical interface (virt-manager).

  • If you prefer a graphical interface over virsh:

    $ sudo apt-get install virt-manager
  • If you are using virsh (note: not applicable if you are using virt-manager), you need the virt-install package to create a new guest and virt-viewer to view the VNC console:

    $ sudo apt-get install python-virtinst virt-viewer
    $ sudo virt-install -n guest_vm -r 512 --disk path=/path/to/store/vm,size=1 -c /path/to/guest_vm.iso -v --virt-type=kvm --connect=qemu:///system --vnc

    The virt-install command creates a VM with the name guest_vm, assigns it 512 MB RAM and creates an associated disk using the arguments provided to --disk path (size 1GB here). For installation, it makes use of the ISO file provided. This ISO may be any OS image that you may have downloaded or generated. The -v option instructs to make the guest 'fully' virtualized with the 'kvm' hypervisor and start the VNC console. There are a multitude of options (like setting the network etc) and you may want to check the man page.

  • In virt-manager (note: not applicable if you are using virsh), go to File > Add Connection and set the Hypervisor as Qemu (or KVM) and save the connection.

    Virt-manager screen

    Then, let's create a guest VM. Right click on the just created connection in the host summary window and create a new VM by right clicking on your connection and selecting 'New'.

    Creating a new VM

    Select the method of your choice to install a new VM. I selected the Local install media (ISO image or CDROM) option, set the name of the VM as guest_vm and provided the path to the ISO image in the next screen. The steps 3 and 4 are fairly straightforward and you can safely use the defaults. In the step 5, you may modify the network (the default should be fine) and have the ability to customize the hardware emulated by the hypervisor. I prefer not to fiddle with those and click on Finish and wait for the VM to get created.

  • Now we are done creating a new guest VM and installing it. We can start the guest using virsh (note: Not applicable if you are using virt-manager):

    $ virsh start guest_vm
    $ virt-viewer -c qemu:///system guest_vm
  • If you want to start the VM using virt-manager, right click on the VM and click on Run.

    Running a VM
  • After running the VM, right click on the running VM and select Open to view the console.

    Running a VM

So now we have guest VMs running and a way to install an OS of our choice. For example, you can install the LTS release of ubuntu in a VM and use it for development or Windows XP (or Windows 7) to test against Internet Explorer or create one VM per project and manage your dependencies easily.