Overview

Teaching: 10 min
Exercises: 5 min

Questions

• What is unpacked.cern.ch?

• How can I use unpacked.cern.ch?

Objectives

• Understand how your images can be put on unpacked.cern.ch

As was pointed out in the previous episode, Apptainer uses unpacked Docker images. These are by default unpacked into the current working directory, and the path can be changed by setting the APPTAINER_CACHEDIR variable.

The EP-SFT group provides a service that unpacks Docker images and makes them available via a dedicated CVMFS area. In the following, you will learn how to add your images to this area. Once you have your image(s) added to this area, these images will be automatically synchronized from the image registry to the CVMFS area within a few minutes whenever you create a new version of the image.

Exploring the CVMFS unpacked.cern.ch area

The unpacked area is a directory structure within CVMFS:

ls /cvmfs/unpacked.cern.ch/

gitlab-registry.cern.ch  registry.hub.docker.com

You can see the full directory structure of an image:

ls /cvmfs/unpacked.cern.ch/registry.hub.docker.com/fnallpc/fnallpc-docker:tensorflow-2.12.0-gpu-singularity/

bin   dev          etc   lib    libgpuarray  mnt  proc  run   singularity  sys  usr
boot  environment  home  lib64  media        opt  root  sbin  srv          tmp  var

This can be useful for investigating some internal details of the image.

As mentioned above, the images are synchronized with the respective registry. However, you don’t get to know when the synchronization happened¹, but there is an easy way to check by looking at the time-stamp of the image directory:

ls -l /cvmfs/unpacked.cern.ch/registry.hub.docker.com/fnallpc/fnallpc-docker:tensorflow-2.12.0-gpu-singularity

lrwxrwxrwx 1 cvmfs cvmfs 79 Apr 17 19:12 /cvmfs/unpacked.cern.ch/registry.hub.docker.com/fnallpc/fnallpc-docker:tensorflow-2.12.0-gpu-singularity -> ../../.flat/7b/7b4794b494eaee76f7c03906b4b6c1174da8589568ef31d3f881bdf820549161

In the example given here, the image has last been updated on August 17th at 13:54.

Adding to the CVMFS unpacked.cern.ch area

You can add your image to the unpacked.cern.ch area by making a merge request to the unpacked sync repository. In this repository there is a file called recipe.yaml, to which you simply have to add a line with your full image name (including registry) prepending https://:

    - 'https://registry.hub.docker.com/fnallpc/fnallpc-docker:tensorflow-2.12.0-gpu-singularity'

As of 14th February 2020, it is also possible to use wildcards for the tags, i.e. you can simply add

    - 'https://registry.hub.docker.com/fnallpc/fnallpc-docker:*'

and whenever you build an image with a new tag it will be synchronized to /cvmfs/unpacked.cern.ch.

Image removal

There is currently no automated ability to remove images from CVMFS. If you would like your image to be permanently removed, contact the developers or open a GitLab issue.

Running Apptainer using the unpacked.cern.ch area

Running Apptainer using the unpacked.cern.ch area is done using the same commands as listed in the previous episode with the only difference that instead of providing a docker:// image name to Apptainer, you provide the path in /cvmfs/unpacked.cern.ch:

apptainer exec --nv -B `readlink $HOME` -B /uscms_data -B /cvmfs /cvmfs/unpacked.cern.ch/registry.hub.docker.com/fnallpc/fnallpc-docker:tensorflow-2.12.0-gpu-singularity /bin/bash

Now you should be in an interactive shell almost immediately without any image pulling or unpacking.

Note

Mind that you cannot change/write files into the container file system with Apptainer. If your activity will create or modify files in the container you will need to write those files to EOS or a mounted directory.

Where to go from here?

Knowing that you can build images on your local machine, Docker Hub, GitHub, or GitLab and have them synchronized to the unpacked.cern.ch area, you now have the power to run reusable and versioned stages of your analysis. While we have only run these containers locally, you can run them on the batch system, i.e. your full analysis in containers with effectively only advantages.

Key Points

• The unpacked.cern.ch CVMFS area provides a very fast way of distributing unpacked docker images for access via Apptainer.

• Using this approach you can run versioned and reusable stages of your analysis.