Unifying filesystems with union mounts
Unification of filesystems is the concept of mounting several filesystems
on a single mount point, with the resulting mount showing the
logical combination of all the filesystems. Traditionally, when a
filesystem is mounted on a directory, the existing contents of the
directory are masked, and the content of the latest mounted
filesystem is shown. These masked files are available only after the
mounted filesystem is unmounted. Even though these files exist, they
are inaccessible to the user. Union mount overcomes this by
providing access to all directories and files present in the
directory, even after a mount.