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Containers and running legacy code

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Learning Objectives

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  • Understand how to use containers to run code requiring older operating systems

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When running code at LHCb there are several scenarios where a user may have to run legacy code intended for an older OS. +The most recent operating systems used is Redhat Enterprise Linux 9 (RHEL9). +There are however, different alternatives to this OS which are roughly equivalent e.g. AlmaLinux 9, Rocky Linux 9.

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There are a few situations where you may need to run legacy code e.g.

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  • Running code from an older analysis

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  • Often times the code used by the Simulation working group requires older operating systems as well

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The previously-used operating systems used by lxplus are listed below. +If not RHEL9, the OS you need will almost certainly be one of thse two:

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  • Centos7 - Retired in 2023

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  • Scientific Linux CERN 6 (SLC6) - Retired in 2018

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Running simulation

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The simulation framework is a common place where older operating systems are needed these are:

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 | Simulation framework | Required OS |
+ | -------------------- | ----------- |
+ | Sim10                | Centos7     |
+ | Sim09                | SLC6         |
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When running legacy code, it is not necessary to find a machine which runs the older OS nor is it necessary to make a virtual machine using the older OS. Instead we can use containers! +Containers are virtualisation software which will allow for multiple operating systems to be run on the same machine, so in this case a machine running RHEL9 can instead use another OS for a while as if it were just another environment.

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The software used on lxplus to jump from one OS to another is called apptainer. To be able to run centos7 software on a RHEL9 machine the following command can be used and adapted:

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apptainer exec --env LBENV_SOURCED= -B /afs/cern.ch/user -B /afs/cern.ch/work -B /cvmfs -e /cvmfs/lhcb.cern.ch/containers/os-base/centos7-devel/prod/amd64 bash --rcfile /cvmfs/lhcb.cern.ch/lib/LbEnv
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We can break down this command in the following way:

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 | Command                                | Explanation                                                     |
+ | -------------------------------------- | --------------------------------------------------------------- |
+ | Apptainer                              | The command being run.                                          |
+ | exec -e <the centos7 environment> bash | Run a bash session with centos7.                                |
+ | -B <directory>                         | These are 'binds', any directory (and its subdirectories) which |
+ |                                        | should be visible within the container need to be specified.    |
+ | --rcfile <group_login>                 | Specifies that LbEnv should be sourced.                         |
+ | --env LBENV_SOURCED=                   | Resets the LBENV_SOURCED environment variable to reload LbEnv   |
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The full list of available platforms is listed here[^1]:

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| Platform | Location                                                        |
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+| slc5     | /cvmfs/lhcb.cern.ch/containers/os-base/slc6-devel/prod/amd64    |
+| slc6     | /cvmfs/lhcb.cern.ch/containers/os-base/slc6-devel/prod/amd64    |
+| centos7  | /cvmfs/lhcb.cern.ch/containers/os-base/centos7-devel/prod/amd64 |
+| el9      | /cvmfs/lhcb.cern.ch/containers/os-base/alma9-devel/prod/amd64   |
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Checking your OS

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At any point the OS you are using can be checked with the command cat /etc/system-release

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  • Try it now both in a RHEL9 environment and a singularity running Centos7 to see the operating system change!

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Warning: minimal use and stability

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One thing to bear in mind about using containers is that they will be less stable than the default OS on your machine. +For this reason it is recommended to only use the containers to run code requiring that OS and do everything else (e.g. git stuff) outside of the container. +This is especially true for containers of older systems such as SLC5 and SLC6.

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[^1]: It’s also possible to get an el9 (i.e. RHEL9) platform. This allows a system running a different linux distro locally (e.g. Ubuntu, Fedora, Arch, …) to run software compiled for EL9!

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