Glossary

Key Points

Introduction	Full CMSSW release containers are very big. It is more practical to use light-weight containers and obtain CMSSW via CVMFS. The centrally supported way to run CMSSW in a container is using Apptainer.
Using full CMSSW containers	Standalone CMSSW containers are currently not routinely built due to their size. They need to be built/requested when needed.
Light-weight CMSSW containers	The light-weight CMS containers need to mount CVMFS. They will only work with CVMFS available.
Accessing CVMFS from Docker locally	You can install CVMFS on your local computer. The `cvmfs-automounter` allows you to provide CVMFS to other containers on Linux. Privileged containers can be dangerous. You can mount CVMFS from within a container on container startup.
Using Singularity	Singularity needs to be used on LXPLUS. CMS Computing provides a wrapper script to run CMSSW in different Linux environments (SLC5, SLC6, CC7, CC8). To run your own container, you need to run Singularity manually.
Using unpacked.cern.ch	The `unpacked.cern.ch` CVMFS area provides a very fast way of distributing unpacked docker images for access via Singularity. Using this approach you can run versioned and reusable stages of your analysis.

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