Kernel Korner - Unionfs: Bringing Filesystems Together

For ease of management, it can be useful to keep related but different sets of files in separate locations. Users, however, often prefer to see these related files together. In this situation, unioning allows administrators to keep such files separate physically, but to merge them logically into a single view. A collection of merged directories is called a union, and each physical directory is called a branch. As shown in Figure 1, Unionfs simultaneously layers on top of several filesystems or on different directories within the same filesystem. This layering technique is known as stacking (see the on-line Resources for more on stacking). Unionfs presents a filesystem interface to the kernel, and in turn Unionfs presents itself as the kernel's VFS to the filesystems on which it stacks. Because Unionfs presents a filesystem view to the kernel, it can be employed by any user-level application or from the kernel by the NFS server. Because Unionfs intercepts operations bound for lower-level filesystems, it can modify operations to present the unified view. Unlike earlier stackable filesystems, Unionfs is a true fan-out filesystem; it can access many underlying branches directly.

Figure 1. A union consists of several underlying branches, which can be of any filesystem type.

In Unionfs, each branch is assigned a precedence. A branch with a higher precedence overrides a branch with a lower precedence. Unionfs operates on directories. If a directory exists in two underlying branches, the contents and attributes of the Unionfs directory are the combination of the two lower directories. Unionfs automatically removes any duplicate directory entries, so users are not confused by duplicated filenames or directories. If a file exists in two branches, the contents and attributes of the Unionfs file are the same as the file in the higher-priority branch, and the file in the lower-priority branch is ignored.

As a concrete example, assume that we unify two directories, /Fruits and /Vegetables:

$ ls /Fruits
Apple Tomato
$ ls /Vegetables
Carrots Tomato
$ cat /Fruits/Tomato
I am botanically a fruit.
$ cat /Vegetables/Tomato
I am horticulturally a vegetable.

To use Unionfs, you first need to compile the Unionfs module and load it into the kernel. Next, like any other filesystem, Unionfs is mounted. Unlike other filesystems, Unionfs does not mount on top of a device; it mounts on top of directories that are specified as a mount-time option. To create a union, we mount Unionfs as follows:

# mount -t unionfs -o dirs=/Fruits:/Vegetables \
> none /mnt/healthy

In this example, the mount option dirs tells Unionfs which directories make up the union. Unionfs does not mount any device, so we use none as a placeholder. Finally, /mnt/healthy is the location of the merged view. Now /mnt/healthy contains three files: Apple, Carrots and Tomato. Because we specified /Fruits before /Vegetables, /mnt/healthy/Tomato contains “I am botanically a fruit.” If we were to reverse the dirs= option, /mnt/healthy/Tomato would contain “I am horticulturally a vegetable.” (which agrees with the 1893 U.S. Supreme Court ruling on the matter).

This process is recursive. If there were a subdirectory of Fruits named Green that contained a file named Lime and a subdirectory of Vegetables also named Green that contained a file named Lettuce, the result would be:

$ ls /mnt/healthy
Apple Carrots Green/ Tomato
$ ls /mnt/healthy/Green
Lime Lettuce

Unionfs can be applied in several ways. Simple examples include unifying home directories from multiple servers or merging split ISO images to create a unified view of a distribution. In a similar vein, Unionfs, with copy-on-write semantics, can be used to patch CD-ROMs, for source code management or for snapshotting.

Often a single client machine mounts home directories from several different NFS servers. Unfortunately, each server has a distinct mountpoint, which is inconvenient for users. It would be ideal if all home directories were available from the same place (/home for example). Some automounters use symbolic links to create the illusion of a union. With Unionfs, these links are not necessary. The separate exported directories simply can be unified into a single view. Assume we have two filesystems, one mounted on /alcid and the other mounted on /penguin. We can unify them into /home as follows:

# mount -t unionfs -o dirs=/alcid,/penguin \
> none /home