Cache docker images on Travis CI

Question

Is it possible to cache docker images on Travis CI? Attempting to cache the /var/lib/docker/aufs/diff folder and /var/lib/docker/repositories-aufs file with cache.directories in the travis.yml doesn't seem to work since they require root.

Answer 1

From a Docker perspective, I think the best way you could do this (without the possibility of running a network local registry) is save the Docker image and cache the exported tar ball. You would need to load that at the start rather than pull an image. This way you're not messing with docker storage implementations.

install:
- docker pull busybox
- docker save busybox | gzip > docker/busybox.tar.gz
cache:
  directories:
  - docker

You would then need to load the cached image before your Travis run.

before_script:
- gzip -dc docker/busybox.tar.gz | docker load

The bit I'm not clear on for Travis, is if you need to stop it from running the install step after the first time. You don't want Travis pulling and exporting the image each time once it's cached. I'm not sure if having the cache directive automatically does that for you?

The main question then is whether this is actually going to be any quicker than pulling the image or not:

The caching tars up all the directories listed in the configuration and uploads them to S3, using a secure and protected URL, ensuring security and privacy of the uploaded archives.

Note that this makes our cache not network-local, it’s still bound to network bandwidth and DNS resolutions for S3. That impacts what you can and should store in the cache. If you store archives larger than a few hundred megabytes in the cache, it’s unlikely that you’ll see a big speed improvement.

You might just be adding overhead. As the Docker registry is backed by Cloudfront, Travis is already pulling compressed images from local, or at least close Amazon infrastructure. Maybe ask them for the feature to cache Docker images natively, similar to what they do for apt packages, although it doesn't sound hopeful.

Answer 2

Have a look at what circleci recommends: https://circleci.com/docs/docker/#caching-docker-layers.

It should be easy to combine docker save/docker load with the directory caching provided by travis.