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What Garbage Collection Does
"Garbage collection deletes blobs which no manifests reference. Manifests and blobs which are deleted by their digest through the Registry API will become eligible for garbage collection, but the actual blobs will not be removed from storage until garbage collection is run.
How Garbage Collection Works
Garbage collection runs in two phases. First, in the 'mark' phase, the process scans all the manifests in the registry. From these manifests, it constructs a set of content address digests. This set is the 'mark set' and denotes the set of blobs to not delete. Secondly, in the 'sweep' phase, the process scans all the blobs and if a blob's content address digest is not in the mark set, the process will delete it.
Note
You should ensure that the registry is in read-only mode or not running at all. If you were to upload an image while garbage collection is running, there is the risk that the image's layers will be mistakenly deleted, leading to a corrupted image.
This type of garbage collection is known as stop-the-world garbage collection. In future registry versions the intention is that garbage collection will be an automated background action and this manual process will no longer apply.
How to Run
You can run garbage collection by running
docker run --rm registry-image-name garbage-collect /etc/docker/registry/config.yml
Additionally, garbage collection can be run in dry-run mode, which will print
the progress of the mark and sweep phases without removing any data.