The contents of the /var/lib/docker directory vary depending on the driver Docker is using for storage.
When you will fall back to overlay, overlay2, btrfs, devicemapper or zfs depending on your kernel support. In most places, this will be aufs but the RedHat went with device mapper.
You can manually set the storage driver with the -s or --storage-driver= option to the Docker daemon.
- /var/lib/docker/{driver-name} will contain the driver specific storage for contents of the images.
- /var/lib/docker/graph/<id> now only contains metadata about the image, in the json and layersize files.
In the case of aufs:
- /var/lib/docker/aufs/diff/<id> has the file contents of the images.
- /var/lib/docker/repositories-aufs is a JSON file containing local image information. This can be viewed with the command docker images.
In the case of device-mapper:
- /var/lib/docker/devicemapper/devicemapper/data stores the images
- /var/lib/docker/devicemapper/devicemapper/metadata the metadata
Note these files are thin provisioned "sparse" files so aren't as big as they seem.
Hope this answer helps you!