Sunday, 23 September 2018

Cleaning up old Docker images in Jenkins

When using Jenkins for building our Docker images we will start suffering problems with the high disk usage because the images are kept stored and they are not deleted by default. The simplest solution would be to delete the image from disk as part of the Jenkins job/pipeline, once the image has been pushed to our Docker Registry.

The major problem of this approach is that on every build triggered, Docker will have to build the new image from scratch and it will not be able to leverage on the Docker layer caching. As you know, when building a new image each instruction within the Dockerfile is examined, and Docker looks for an existing image in its cache that it can reuse, rather than creating a new (duplicate) image. This implies that the building of new images will probably be much slower. So ideally we would like to keep on disk at least the latest version of our image, so that Docker can reuse previous created layers for building the new image quickly.

As an example, suppose we have a job/pipeline in Jenkins for building our images called "myDockerImage", using the Jenkins build number as image tags.

$ docker images
REPOSITORY                                         TAG                   IMAGE ID            CREATED             SIZE
myDockerImage                                      456                   1fdcfbbb6e95        2 days ago          1.2GB
myDockerImage                                      455                   e7deb97e4677        2 weeks ago         1.2GB
myDockerImage                                      454                   c4297a8dc099        2 weeks ago         1.2GB
myDockerImage                                      453                   36f69b5805d9        2 weeks ago         1.2GB
...                                                ...                   ...                 ...                 ...
centos                                             6.8                   d63aca2e7714        6 weeks ago         195MB
nginx                                              1.15.2-alpine         36f3464a2197        8 weeks ago         18.6MB
postgres                                           alpine                e6c5e6a76255        8 months ago        38.2MB
rabbitmq                                           3.6.14-alpine         5a9e735a5ad8        8 months ago        38.9MB
openjdk                                            8u151-jre-alpine3.7   b1bd879ca9b3        8 months ago        82MB
mysql                                              5.7                   c356247174ed        2 months ago        372MB
varnish                                            latest                ec0406c252f9        3 months ago        161MB
alpine                                             3.7                   3fd9065eaf02        8 months ago        4.15MB
ruby                                               2.3.1                 ffe8239a147c        22 months ago       730MB

As you can see, we have other images stored as well. Now, how could we delete all images of "myDockerImage" except the one we have just created? Let's take a look to the different options to the "docker images" command:

$ docker images --help

Usage: docker images [OPTIONS] [REPOSITORY[:TAG]]

List images

Options:
  -a, --all             Show all images (default hides intermediate images)
      --digests         Show digests
  -f, --filter filter   Filter output based on conditions provided
      --format string   Pretty-print images using a Go template
      --no-trunc        Don't truncate output
  -q, --quiet           Only show numeric IDs

The first step would be to list only the desired image (myDockerImage):

$ docker images myDockerImage
REPOSITORY                                         TAG                   IMAGE ID            CREATED             SIZE
myDockerImage                                      456                   1fdcfbbb6e95        2 days ago          1.2GB
myDockerImage                                      455                   e7deb97e4677        2 weeks ago         1.2GB
myDockerImage                                      454                   c4297a8dc099        2 weeks ago         1.2GB
myDockerImage                                      453                   36f69b5805d9        2 weeks ago         1.2GB
...                                                ...                   ...                 ...                 ...

Then we could use the filter option (-f) for listing only the images that were built "before" the one we have just built:

$ docker images --filter "before=myDockerImage:456" myDockerImage
REPOSITORY                                         TAG                   IMAGE ID            CREATED             SIZE
myDockerImage                                      455                   e7deb97e4677        2 weeks ago         1.2GB
myDockerImage                                      454                   c4297a8dc099        2 weeks ago         1.2GB
myDockerImage                                      453                   36f69b5805d9        2 weeks ago         1.2GB
...                                                ...                   ...                 ...                 ...

Then we will use the option -q for listing only the numeric IDs, and the command "docker rmi" for deleting the images in the list.
$ docker rmi $(docker images -q --filter "before=myDockerImage:456" myDockerImage)

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