Simple definition
OverlayFS combines lower and upper filesystem layers, enabling copy-on-write views used by containers.
Purpose and use
OverlayFS presents a merged directory tree from filesystem layers, often for container images.
How does it work?
An upper layer receives changes; modifying a lower-layer file may trigger copying into that upper layer.
Example and verification
To locate a change, identify the relevant lower layers, upper layer and merged mount.
Points to watch
A visible deletion may use a marker without deleting the lower-layer file. Backing up only one layer may be insufficient.
In short
The merged view differs from the data stored in each individual layer.