Sorry for late report, things are not going as I planned. In order to implement tree manipulation or copy-on-write function, I must first have a blocks/extents allocator that works well. Also, those things need to handle transactions as well.
This is a companion discussion topic for the original entry at https://www.haiku-os.org/blog/hyche/2017-07-07_gsoc_2017_-_btrfs_write_supports_week_6_7_8/