First off, I've seen another issue that talks about not deleting image layers. Because of this, I figured I'd write my own image layer cleanup algorithm. The algorithm is simple: Walk the repositories using _index_images and mark image ids to keep. Once you're done scanning the repositories, delete all image layers that weren't marked to be kept.

I found out there's a problem with this, however: It seems that when you delete a tag in a repository, the repository's _index_images file does not get updated to remove any image layers that were only used by that tag.

It seems that since _index_images is sort of the single source for seeing what image layers a repository uses, that it should be updated accordingly whenever a tag is deleted.

I realize that doing this would increase the time it takes to do a tag delete, but as it stands currently, _index_images cannot be trusted to be the definitive list of image layers used in the repository.