10.08.2015: CernVM 3.5


CernVM 3 is a virtual machine image based on Scientific Linux 6 combined with a custom, virtualization-friendly Linux kernel.

CernVM 3 is based on the µCernVM bootloader. Its outstanding feature is that it does not require a hard disk image to be distributed (hence “micro”). Instead it is distributed as a read-only image of ~20MB containing a Linux kernel and the CernVM-FS client. The rest of the operating system is downloaded and cached on demand by CernVM-FS. The virtual machine still requires a hard disk as a persistent cache, but this hard disk is initially empty and can be created instantaneously, instead of being pre-created and distributed.

CernVM 3 comes with a C++11 compiler, a Go compiler, an Erlang interpreter, GNUplot, ROOT, R, octave, LaTeX, scipy and numpy, and many other useful packages. For distributed computing, CernVM 3 comes with tools such as Condor, Ganglia, Squid, XrootD, Puppet, CernVM CoPilot, Parrot and Workqueue/Makeflow, and others. In order to manage your virtual machines in the cloud, CernVM 3 comes with the cloud management utilities for OpenStack (nova, glance), Amazon EC2 (ec2-… and euca-…), Google Compute Engine (gcutil, gsutil, gcloud), and Microsoft Azure (azure).

If you believe an important package is missing, please let us know!

Update from CernVM 3.1 , CernVM 3.2, CernVM 3.3, or CernVM 3.4

In order to update from CernVM 3.1, CernVM 3.2, CernVM 3.3, or CernVM 3.4 run

sudo cernvm-update -k
sudo cernvm-update -a

If you have a running CernVM 3.4, there is no need to update the kernel. It is sufficient to run sudo cernvm-update -a. If you update from an earlier version, a lockstep update is required of both the µCernVM bootloader and the operating system. Unlike the CernVM security updates, this is a minor release. We recommend to use your hypervisor to make a VM snapshot before you update.

Note on docker: after upgrading from a version < CernVM 3.4, please add your user account manually to the docker group through

sudo usermod -aG docker <USER NAME>

Changes to compared to CernVM 3.4

  • A number of base packages are updated to Scientific Linux 6.7
  • Support for Microsoft Azure
  • Support for CloudStack
  • Support for glideinwms-vm contextualization
  • Java 7 and Java 8 runtime environments
  • GUI fix: don’t show removable devices on the Xfce4 desktop
  • Minor kernel update to 3.18.20
  • A number of security fixes, mostly as a result of Scientific Linux 6.7 release: ansible, autofs, bind, cryptopp, curl, gnutls, grep, java-1.6.0-openjdk, firefox, httpd, libunwind, libuser, libxml2, net-snmp, ntp, qemu-img, php, python, python-keyring, python-pip, sudo, wpa_supplicant, xrdp

Enforce booting a previous version of CernVM

Booting a previous version of CernVM can be enforced using the following contextualization snippet:


The VERSION corresponds to the version of the cernvm-system RPM. For an interactive virtual machine, hit <TAB> in the early boot menu and then e to edit the entry. You can change the cvmfs_repository_tag boot parameter from HEAD to VERSION.

Note that previous versions do not contain the latest security fixes.


Download the latest images from our download page. The image is generic and available in different file formats for different hypervisors. The image needs to be contextualized to become either a graphical development VM or a batch virtual machine for the cloud.

Detailed instructions are available for VirtualBox, VMware, KVM, CERN OpenStack, Amazon EC2, Google Compute Engine, and Microsoft Azure.


A CernVM needs to be contextualized on first boot. The process of contextualization assigns a profile to the particular CernVM 3 instance. For instance, a CernVM can have the profile of a graphical VM used for development on a laptop; applying another context let the CernVM become a worker node in the cloud.

The CernVM Online portal lets you define and store VM profiles in one place. Once defined, the VM profiles can quickly be applied to any newly booted CernVM instance using a pairing mechanism on the login prompt. Please visit the following pages for more information about how to create new context templates and pair an instance with given template.

Also check out the CernVM Online Marketplace.

Please find details on the various contextualiztion options on the contextualization page.

Updates and Version Numbers

When booted, CernVM will load the latest available CernVM 3 version and pin itself on this version. This ensures that your environment stays the same unless you explicitly take action. Both the µCernVM bootloader and the CernVM-FS provided operating system can be updated using the cernvm-update script. CernVM machines show an update notification in /etc/motd and in the GUI. The support list will be notified when updates are ready and will post specific instructions for each update.

The CernVM 3 strong version number consists of 4 parts: 3.X.Y.Z. Major version 3 indicates an Scientific Linux 6 based CernVM. Minor version X will be changed when there is a significant change in the set of supported features. “Y” is the bugfix version. “Z” is the security hotfix version; changes in “Z” don’t change the set of packages but provide security fixes for selected packages.

Next steps

Once booted and contextualized, you can use ssh to connect to your virtual machine. SSHFS and shared folders provide you an easy means to exchange files between the host and CernVM.

For storing data and analysis results, we recommend not to use the root partition. Instead, attach a second hard drive to the virtual machine or use shared folders. This way, you can move data between virtual machines and the data remains intact even in case the virtual machine ends up in an unsuable state.


In order to start a stand-alone ROOT, click on the tree logo in the middle of the application launcher bar. In non-graphical mode, use module load ROOT to set up the ROOT environemnt. Afterwards you can just use root. If you want to clean the environment from that particular version of ROOT, use module unload ROOT.

C++11 Compiler

CernVM 3 comes with a C++11 compliant gcc 4.8 compiler that can be used in lieu of the Scientific Linux 6 system compiler. In order to enable it, run source /opt/rh/devtoolset-2/enable. Apart from gcc 4.8, this also provides matching binutils and a matching gdb and Valgrind.

Modern LaTeX

By default, CernVM uses the TeXlive 2007 that comes with Scientific Linux 6. A newer TexLive 2014 distribution is installed in /cvmfs/sft.cern.ch. In order to activate it, run module load texlive (and module unload texlive to clean the environment).


CernVM can run docker containers. In order to enable docker support, run sudo /sbin/service docker start. In order to enable docker on boot, run sudo /sbin/chkconfig docker on.


By default, CernVM comes with a Java 6 environment. Java 7 and Java 8 environments are available. In order to change the default Java version, use update-alternatives --config java.

Single Sign On

You can get a Kerberos token with kinit. With the token, you can login to lxplus and work with subversion repositories without the need to provide a password. Note that due to DNS issues, this feature is not available in VirtualBox.


CernVM has AFS installed but disabled by default. In order to start AFS, you’d need to do

sudo mkdir /afs
sudo /sbin/chkconfig afs on
sudo /sbin/service afs start

Once mounted, you can get an AFS token using kinit followed by aklog. In our experience, AFS works poorly behind NAT. So AFS might be helpful for CernVM on CERN OpenStack but it is not really an option for VMs on the laptop. Instead, shared folders or sshfs can provide an easy way to map directory trees from the host inside the guest.

Swap Space

By default, CernVM has no swap space enabled. The following commands creates a 2G swap file

sudo fallocate -l 2G /mnt/.rw/swapfile
sudo chmod 0600 /mnt/.rw/swapfile
sudo mkswap /mnt/.rw/swapfile
sudo swapon /mnt/.rw/swapfile

If a file /mnt/.rw/swapfile exists, it will picked up automatically on boot as a swap space. In order to activate a swap space through contextualization, add to your amiconfig user data


where <SIZE> can be anything understood by fallocate -l or it can be auto, in which case CernVM uses 2G/core.


CernVM 3 supports the Makeflow workflow engine. Makeflow provides an easy way to define and run distributed computing workflows. The contextualization is similar to condor. There are three parameters:

catalog_server=hostname or ip address
workqueue_project=project name, defaults to "cernvm" (similar to the shared secret in condor)
workqueue_user=user name, defaults to workqueue (the user is created on the fly if necessary)

In order to contextualize the master node, include an empty workqueue section, like


In order to start the work queue workers, specify the location of the catalog server, like



The plugin will start one worker for every available CPU.

Once the ensemble is up and running, makeflow can make use of the workqueue resources like so

makeflow -T wq -a -N foobar -d all -C makeflow.example

Note that your cloud infrastructure needs to provide access to UDP and TCP ports 9097 on your virtual machines.

Known Issues

  • If the keyboard layout changes, run sudo system-config-keyboard to bring it back to match your keyboard


In case you cannot login (any more) to your virtual machine, even though the machine was properly contextualized, you can boot CernVM in “debug mode”. In the early boot menu, select the “Debug” entry. This enables kernel debug messages and pauses the boot process just before the µCernVM bootloader hands over to the operating system. Here, type reset_root_password followed by ENTER and Ctrl+D. Once booted, you can then login as root with password “password”.

If you experience hangs of the virtual machine after changing location, try to reset the network by sudo /sbin/service network restart.

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