What a clever hack. The mini_fo filesystem allows writing to read-only file systems. Well, not really, of course. Conceptually, it's like using a transparent overlay to draw something on a slide projector. You aren't really writing to the read-only file system, but that's the apparent affect for anyone reading it. This is sort of the inverse of a Snapshot.
The original intent was to allow upgrades to embedded systems, but it could also be used for Kiosks and test systems where you want to maintain a certain image but test changes to it easily. Dry run testing of software updates also seems like a great application for this. Such a neat idea; I'm surprised it wasn't done earlier.
In practice, you mount an already mounted filesystem at a new mount point as a mini_fo filesystem. You then point reads and writes to that directory. Pretty simple.
Read more about it at https://www.denx.de/wiki/bin/view/Know/MiniFOHome.
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