Advanced multi-layered unification filesystem (AUFS) is a union filesystem sometimes used in platform as a service environments to merge distinct directory hierarchies into a single directory. This may be done to consolidate a large repository without using a redundant array of independent disks (RAID) or similar techniques.
AUFS can also be used to convert a large read-only file system into one with a writeable area that allows and stores changes to the existing data. In the second use case, this type of writeable system can be applied separately for different apps. Such an approach supports efficient storage by using a single base image of the read-only file system across multiple applications -- each with its own, separate read/write capability -- without creating a new image of the entire filesystem for each application. Reusing a base image reduces the I/O load and the space consumed in the buffer cache because there are no multiple versions of the filesystem to be cached.
Changes to the base image using the AUFS approach are instantly available systemwide, bypassing the potential drawbacks of using snapshots and copy-on-write devices to create clones. AUFS also permits changes in a private layer without impacting the base read-only filesystem.