Example of simple filesystem

[Jakub Rajman]

Hello,

I am going to write my own simple file system and run it under the
FreeBSD OS. I'm looking for some inspiration in existing file systems
(UFS2, msdosfs,...), but they are too complex and complikated.
So I would like to ask, if exists some simple "experimental"
filesystems? Some workable (and mountable) examples with their source
codes.

Thank you for any help