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CernVM-FS Graph Driver Plugin for Docker

The CernVM-FS graph driver plugin for Docker provides a dockerized CernVM-FS client that can be used by the Docker daemon to access and store container images that reside in an extracted form on a CernVM-FS repository. Because CernVM-FS downloads the files of a container image only when accessed and because typically very little of a container image is accessed at runtime, the CernVM-FS graph driver can remove the bottleneck of distributing (large) container images to (many) nodes.

The CernVM-FS graph driver can run any normal image from a Docker registry. Additionally, it can run so called Thin Images. A thin image is like a symbolic link for container images. It is a regular, very small image in the registry. It contains a single file, the thin image descriptor, that specifies where in a CernVM-FS repository the actual image contents can be found. The docker2cvmfs utility can be used to convert a regular image to a thin image.

Comparision between regular container images and thin images

Requirements

The graph driver plugin requires Docker version > 17 and a host kernel with either aufs or overlay2 support, which includes RHEL >= 7.3. Please note that on RHEL 7, Docker's data root should reside either on an ext file system or on an xfs file system that is formatted with the ftype=1 mount option.

The Docker graph driver plugin receives its CernVM-FS configuration by default from the Docker host's /etc/cvmfs directory. The easiest way to populate /etc/cvmfs is to install the cvmfs-config-default package (or any other cvmfs-config-... package) on the Docker host. Alternatively, a directory structure resembling the /etc/cvmfs hierarchy can by manually created and linked to the graph driver plugin.

Installation

The folling steps install and activate the CernVM-FS graph driver plugin.

  1. Install the plugin with docker plugin install cvmfs/graphdriver. The command docker plugin ls should now show the new plugin as being activated.

  2. Create or edit the file /etc/docker/daemon.json so that it contains the following content

    {
  "experimental": true,
  "storage-driver": "cvmfs/graphdriver",

  // To change the docker data root to an ext formatted location (remove this line)
  "data-root": "/path/to/ext/mountpoint",

  // Add the following storage option on RHEL 7 (remove this line)
  "storage-opts": [
    "overlay2.override_kernel_check=true"
  ]
}

  3. Restart the Docker daemon with systemctl restart docker.

  4. Test the new plugin with a normal image

    docker run -it --rm ubuntu /bin/bash

    and with a thin image

    docker run -it --rm cvmfs/thin_ubuntu /bin/bash

In order to get debugging output, add "debug": true to the /etc/docker/daemon.json file.

Location of the Plugin Configuration

By default, the plugin tries to bind mount the host's /etc/cvmfs directory as a source of configuration. Other locations can be linked to the container by running

docker plugin set cvmfs/graphdriver cvmfs_ext_config="/alternative/location"
docker plugin set cvmfs/graphdriver minio_ext_config="/alternative/location"

Installation from a Plugin Tarball

Instead of installing the plugin from the Docker registry, it can be installed directly from a tarball. To do so, download and untar a graph driver plugin tarball. Run

docker plugin create my-graphdriver cvmfs-graphdriver-plugin-$VERSION
docker plugin enable my-graphdriver

Note: currently, the graph driver name (my-graphdriver) must not contain a colon (:) nor a comma (,). This issue will be fixed in a lalter version.

Conversion of Images

A simple way to ingest docker images inside a cvmfs repository is available through a small utility docker2cvmfs.

At the moment is possible to directly download the executable: docker2cvmfs v0.3

docker2cvmfs provides different commands to manipulate docker images but the simplest way is to use the make-thin sub-command.

This sub-command expects to find on the host machine a recent version of cvmfs that support the ingest commands.

Invoking the help of the subcommand docker2cvmfs make-thin --help explain what options are available and how to use them.

Below we provide a complete example on how to use docker2cvmfs to convert the docker image of Redis into a thin image.

Assuming a cvmfs repository called example.cern.ch is already in place:

./docker2cvmfs make-thin --input-reference library/redis:4 --output-reference thin/redis:4 --repository example.cern.ch

The utility takes as input the reference (library/redis:4) to the image to ingest into cvmfs along with the reference to associate to the new thin image (thin/redis:4) and the repository where we want to store the several layers (example.cern.ch).

The utility download every layer that compose the image, store them into the repository, create then new thin image and import that into docker.

By default the layers are stored into the layers/ subdirectory of the repository, this can be modified using the --subdirectory parameters.

The images are downloaded, by default, from the official docker hub registry, this can be modified as well using the --registry parameter.