Overlayfs@Docker

https://docs.docker.com/storage/storagedriver/overlayfs-driver/#how-the-overlay2-driver-works

OverlayFS layers two directories on a single Linux host and presents them as a single directory. These directories are called layers and the unification process is referred to as a union mount. OverlayFS refers to the lower directory as lowerdir and the upper directory a upperdir. The unified view is exposed through its own directory called merged.

The overlay2 driver natively supports up to 128 lower OverlayFS layers. This capability provides better performance for layer-related Docker commands such as docker build and docker commit, and consumes fewer inodes on the backing filesystem.