With FUSE it is possible to implement a fully functional filesystem in a userspace program. There's also companion library: libfuse. This library along with utilities is available from the FUSE homepage: <http://fuse.sourceforge.net/> See <file:Documentation/filesystems/fuse.txt> for more information. See <file:Documentation/Changes> for needed library/utility version. If you want to develop a userspace FS, or if you want to use a filesystem based on FUSE, answer Y or M.