AUFS

AUFS, short for Another Union File System, is a stackable Linux filesystem that allows combining several directories or filesystems into a single coherent view. It presents a unified namespace by layering read-only lower branches beneath a writable top branch. Applications can mount multiple directories as branches (layers) and access them as one.

AUFS supports an arbitrary number of branches, and when a file is modified, it is copied into the top writable layer, implementing copy-on-write semantics. It also provides whiteouts, which are special entries used to mark deletions in the lower layers, and opaque directories to hide lower layers entirely under a given directory. This enables a flexible, non-destructive way to build up complex filesystem images.

In practice, AUFS has been widely used as a storage driver in container platforms (notably Docker) and in live and embedded Linux systems where layering of filesystem images is beneficial. It predates OverlayFS in the Linux kernel and provided a mature multi-layer capability before OverlayFS gained acceptance.

Status: AUFS is implemented as a kernel patch and is not part of the mainline Linux kernel. As OverlayFS became the standard union filesystem in mainline kernel space, many distributions archived or deprecated AUFS in favor of OverlayFS. Nevertheless, AUFS remains available in some distributions and community projects, and it is still referenced in historical discussions of container storage technology.