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auf (in version 1 Another Unionfs , from version 2 advanced multi layered unification filesystem ) is an overlay file system that is required for the (apparent) writing of data on non-writable data carriers (such as CD-ROMs and DVDs ). For this purpose, at least two file systems are placed on top of one another. A writable file system is placed over the non-writable one. If a file is now to be read, an attempt is first made to read it on the writable file system. If it is not there, it is read from the non-writable file system below. Write access is always made to the writable file system.
auf is a spin-off from UnionFS , which was completely redeveloped by Junjiro Okajima, who has also delivered many bug fixes for UnionFS . Is used on z. B. in Knoppix (from version 5.1.0) and replaces UnionFS there due to instabilities.