Unionfs is a stackable file system that virtually merges a set of directories (called branches) into a single logical view. Each branch is assigned a priority and may be either read-only or read-write. When the highest priority branch is writable, Unionfs provides copy-on-write semantics for read-only branches. These copy-on-write semantics have lead to widespread use of Unionfs by LiveCD projects including Knoppix and SLAX. In this paper we describe our experiences distributing and maintaining an out-of-kernel module since November 2004. As of March 2006 Unionfs has been downloaded by over 6,700 unique users and is used by over two dozen other projects. The total number of Unionfs users, by extension, is in the tens of thousands.

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