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Is the docker registries garbage collection garbage?

Step 1) Run garbage collection on my registry

docker-compose exec registry bin/registry garbage-collect /etc/docker/registry/config.yml
....
4005 blobs marked, 0 blobs and 0 manifests eligible for deletion

So everything seems used in some image:tag.

Step 2) Check size

du -sch $(docker volume inspect registry_registry -f "{{.Mountpoint}}")
50G     /var/lib/docker/volumes/registry_registry/_data

This is big, for the amount of images in there. It's 73 images, all originating in the same base images. Everything is either alpine or debian.

Step 3) Build a list of all images and all tags, download everything

python list-all-repos.py --registry ... --ask-pass > all-repos
for img in $(cat all-repos); do docker pull $img; done

(See below for script source)

Step 4) Throw away all registry data

docker-compose down -v && docker-compose up -d

Step 5) Upload everything again

for img in $(cat all-repos); do docker push $img; done

Now that everything is up again, I checked that all images are there again. Just ran the script again and diffed the output. No differences.

Step 6) Check size again

du -sch $(docker volume inspect registry_registry -f "{{.Mountpoint}}")
6.4G    /var/lib/docker/volumes/registry_registry/_data

...garbage-collect
473 blobs marked, 0 blobs and 0 manifests eligible for deletion

So, what am I not getting? Shouldn't GC already have thrown away everthing not referenced by some image:tag?

The script to list all images

This depends on DXF

"""Print all images and tags from given docker registry."""
import os
import argparse
import getpass

from dxf import DXF, DXFBase


def _get_arguments():
    parser = argparse.ArgumentParser(
        description="Print all images and tags from registry."""
    )
    parser.add_argument('--registry', dest='registry')
    parser.add_argument('--username', dest='username',
                        help="Defaults to $REGISTRY_USERNAME")
    parser.add_argument('--password', dest='password',
                        help="Defaults to $REGISTRY_PASSWORD")
    parser.add_argument('--ask-pass', dest='ask_pass', action='store_true',
                        help='Interactively ask for password')
    return parser.parse_args()


def main(registry, username, password):
    """Print all images and tag."""
    regbase = DXFBase(registry)
    regbase.authenticate(username, password)

    for repo in regbase.list_repos():
        regrepo = DXF(registry, repo)
        regrepo.authenticate(username, password)
        aliases = regrepo.list_aliases()
        for alias in aliases:
            print(f"{registry}/{repo}:{alias}")


if __name__ == "__main__":
    args = _get_arguments()
    _username = (args.username if args.username else
                 os.environ.get('REGISTRY_USERNAME'))
    if args.ask_pass:
        _password = getpass.getpass(f"{args.registry} password: ")
    else:
        _password = (args.password if args.password else
                     os.environ.get('REGISTRY_PASSWORD'))
    main(
        args.registry,
        _username,
        _password,
    )