What is mini_fo?
mini_fo is a virtual kernel file system that can make read-only file systems writable. This is done by redirecting modifying operations to a writeable location called "storage directory", and leaving the original data in the "base directory" untouched. When reading, the file system merges the modifed and original data so that only the newest versions will appear. This occurs transparently to the user, who can access the data like on any other read-write file system.
Base and storage directories may be located on the same or on different partitions and may be of different file system types. While the storage directory obviously needs to be writable, the base may or may not be writable, what doesn't matter as it will not be modified anyway.
mini_fo was primarily developed for use in embedded systems and therefore was designed to be lightweight in terms of module size, memory usage and storage usage. Nevertheless it has proved useful for various other projects (see below).
mini_fo文件系統的知識可以參照一下兩篇文檔，比較清晰。
http://lwn.net/Articles/135283/