Registry mirror disk clanup


In the documentation of the registry as pull-through cache (or mirror), i see the following sentence: “When running as a pull through cache the Registry periodically removes old content to save disk space”.

Does anyone know what it means? Specifically: What is this old content? Is it images? And, how often is it cleaned? it it configurable?

And another question: what is the best way to know what images exist on the mirror? if you simply ask for the list of tags for a given repo through API, the result shows all the tags in DockerHub.