3.1 Interposition API

The mount system call is used to interpose one file system on top

  

Figure: Interposition Resulting in Fan-in or Fan-out

2.6in

Figure: Interposition Resulting in Fan-in or Fan-out

2.2in

of another. Likewise, umount unmounts and hence de-interposes a file system. Mounting can be relative to any file system above or below, so that file systems can be ``stacked'' into a DAG. As an example, suppose that file system X is interposed on file system Y. To create fan-in, file system Z can be mounted above Y. To create fan-out, Z can be mounted below X.

Figure fig-fanning shows what mounts result in a fan-in vs. a fan-out. The information of how many file systems are mounted at a mount point is stored in a private VFS data structure, and is described in Section sec-design-implement-wrapfs-vfs.