I noticed today an alert on alert hosts re: / at warning level, the investigation proceeded as follows:
- find out that /var/lib/docker is using a ton of space
- issue a docker system prune to clear out old images
- in the meantime observe that the reason there are massive data and metadata files is because those are used for devicemapper storage backend
- our docker::engine profile checks if you have enabled overlayfs (default off) or not, and issues a warning()
- the warning is logged server side only, so we never noticed that you are not supposed to run docker::engine without also setting profile::base::overlayfs: true