This chapter will describe the internal structures you must know if you plan to add a new filesystem into OpenBSD kernel. As you know OpenBSD is based upon netBSD now. The filesystem interface has not yet evolved since netBSD fork so it can be considered very old. The interface is an implementation of the vfs interface definition. If you don't know vfs and others filesystems interfaces we recommend that you read Toward a Compatible Filesystem Interface that explain how this interface was designed and why. It's a documentation done by Berkeley BSD team comparing Unix filesystem interfaces and explaining why they used vfs into 4.4BSD.