How to write OOBtree indexing on disk?

I have large information to index and create posting List for retrieval later, I searched and understood that BTree Data structure is suitable to store large data mapping on disk.

In The python Libraries I found Btree 4.08 and read the Document, I used OOBTree module in my code, insert data to OOBtree but I don't know which part of this tree must be stored on disk, or how to store?

Here is link for BTree Python Document:

You may consider ZODB, and its BTree module. ZODB is an object database, providing an efficient way to read and write Python objects and structures to disk.