Pruning of images in the docker daemon's graphdriver storage for devicemapper was added in 3.2. You may still see this message at atomic-openshift-node startup because the node might not have collected data yet, but it should have it eventually (within a few minutes).
https://github.com/google/cadvisor/pull/1070 https://trello.com/c/JCbV4gmQ/200-2-add-devicemapper-thinpool-support-to-cadvisor-for-image-garbage-collection
Not enough information to definitely dup to this bug, but it could be the issue described in 1408309 could be preventing proper garbage collection. The customer ticket has been closed so I'm closing this bug.