When hierarchical filesystems behave as pure trees, it's always possible to simply deallocate the storage of a deleted file, because the file cannot be pointed by more than one entry in the tree.

Unix filesystems, however, do have a way of creating several references to the same file, namely hard links. In order to handle these multiple references (like having a program with multiple variables pointing to the same object) a kind of GC algorithm is used, which is simply a "reference counting GC".

Reference counting is a simple technique which is appropriate in several cases but has it's drawbacks and it's One of these drawbacks can be illustrated by this quote:

"One day a student came to Moon and said: “I understand how to make a better garbage collector. We must keep a reference count of the pointers to each cons.”

Moon patiently told the student the following story:

“One day a student came to Moon and said: ‘I understand how to make a better garbage collector... ”"