A union mount is a filesystem mount in a computer that allows all the different mounts to overlay one another. A regular mount allows each to exist in its own realm. With a union mount, all the files from each mount are accessible. At the same time, some files may be masked if they have the same name as files from the top-most mount, because the top mount takes priority. Another difference between this mount type and others is that the top mount will have read-write access, unlike most mounts, which just have read access.
When a filesystem mount is performed, each mount is positioned in a different place in the computer’s memory, so none of the mounts interferes with another. While this has some advantages, it can make it difficult for users to access the mounts if they need files from several mounts at once. With a union mount, each of the mounts is overlaid on top of one another so, while the mounts exist in different areas of the memory, they are organized in a fashion that enables users to access all of them.
A union mount organizes all the mounts, so all of the files from each mount are accessible. This means that any of the programs, documents or anything else included in any of the mounts is open for users to see. While this may not benefit common users, programmers and users who perform multiple mounts can benefit from this by making it easier to modify files from the mounts, because users will not have to jump around the computer to find the files.
While most of the files in a union mount are not masked, or invisible, a few may be. Some of the files within the different mounts may have the same file name, either by coincidence or because the mounts deal with the same filesystem. If this occurs, then only the files in the top-most mount will appear, while those in the lower mounts will be invisible. It does not matter if the files have different contents; they will still be invisible.
Another advantage to having a union mount is that the top-most mount will have read-write access. Mounts normally only have read access, meaning users can read and look at the files in the mount, but they cannot be changed. With this mount type, users are able to change the files in the top-most mount.