Exploring Docker Image Layers and Size Management

Exploring Docker Image Layers and Size Management

Docker images are built from layers, where each layer represents a set of filesystem changes. The size of an image on disk is the sum of the sizes of its component layers. Docker allows you to commit changes to a running container, creating new image layers.

Remove Git:

Create a new container from the ubuntu-git image and remove Git.

docker run --name ubuntu-git-remove --entrypoint /bin/bash ubuntu-git:latest -c "apt-get remove -y git"

This command runs a container named ubuntu-git-remove from the ubuntu-git:latest image with an entrypoint set to /bin/bash, and removes Git from the container.

Commit the Changes to Create a New Image with Git Removed:

Commit the container to create a new image with Git removed.

docker commit ubuntu-git-remove ubuntu-git:2.0

docker tag ubuntu-git:2.0 ubuntu-git:latest

These commands commit the current state of the ubuntu-git-remove container to a new image named ubuntu-git:removed and reassign the latest tag to this new image.

Check Image Sizes:

Check the sizes of all the images created.

docker images

Expected output:

Notice that even though you removed Git, the image actually same in size. Although you could examine the specific changes with docker diff, you should be quick to realize that the reason for the increase has to do with the union file system.

Remember, UFS will mark a file as deleted by actually adding a file to the top layer. The original file and any copies that existed in other layers will still be present in the image. When a file is deleted, a delete record is written to the top layer, which overshadows any versions of that file on lower layers.

It’s important to minimize image size for the sake of the people and systems that will be consuming your images. If you can avoid causing long download times and significant disk usage with smart image creation, then your consumers will benefit.