Абсолютно всю архитектуру, к которой он просто добавил добавил CoW от Охада Родеха.

https://lwn.net/Articles/342892/

Chris Mason took these COW-friendly btrees and ran with them. Back in the day, Chris worked on Reiserfs, where he learned a lot about what to do and what not to do in a file system. Reiserfs had some cool features - small file packing, btrees for fast lookup, flexible layout - but the implementation tended to be haphazard and ad hoc

The leaves of the btree contain items, which are a combination of keys and data. Similarly to reiserfs, the items and data are packed in extremely space-efficient way: the item headers (that is, the item structure described above) are packed together starting at the beginning of the block, and the data associated with each item is packed together starting at the end of the block. So item headers and data grow towards each other, as shown in the diagram to the right.