GitHub - spotify/docker-gc: INACTIVE: Docker garbage collection of containers and images

I’m from the valley of the heavy heads.

When you type docker images you get a list of docker images on your system. The image itself is basically a tar file with a content hash. It’s cryptographically guaranteed (like git) to be the content you want because of this hash. You have a local image store because it’s much faster to load content locally than over the internet. So, in this way, docker images are a cache like any other cache. $ docker images REPOSITORY TAG IMAGE ID CREATED SIZE <none> <none> 77af4d6b9913 19 hours ago 1.089 GB postgres latest 746b819f315e 4 days ago 213.4 MB Cache Invalidation is Hard What’s the hardest problem with caching? Expiration! So knowing when to “expire” a docker image must be pretty tricky. Spoiler: it is. First, let’s talk about why we’d want to expire or even manage our docker images to begin with. Docker has this image store but in the beginning they didn’t have a clear procedure for what you were supposed to do when you accumulate more and more images. You either fill your physical disk on Linux or fill a virtual disk on Mac/Windows and then docker stops working. There are countless threads about the qcow file but there was never any guidance as to what people were supposed to do to manage the images. Spotify created a helper script called docker-gc to help manage this problem. It’s been archived in favor of docker system prune but this is not a complete solution. Removing images is like expiring the cache, the tricky part is knowing when to do so. When you are building images, docker will print out intermediate steps as SHAs that you can enter and debug. When you tag a SHA hash, you are tagging a <none>. If you docker system prune then you could throw away data you care about and image prune is not much better. More so, you are blowing away your cache so builds will take longer. Spotify’s docker-gc let you specify images you want to keep which is useful for the previous examples when you are making images. Sometimes, people suggest cron’ing the prune so the disk ne