8.5.1 Mounting

Consider first the two file systems X and Y, depicted in Figure fig-vnode-fs-xyz. In this figure, the numbers near the node names represent the file/inode/vnode numbers of that file or directory within that particular file system. For example ``X5'' refers to the vnode of the directory /usr/local on file system X.

  

Figure: File System Z as Y mounted on X

Figure: File System Z as Y mounted on X

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Let's also assume that X is a UFS (local) file system, and that Y is the /usr file system available on a remote file server named ``titan.'' We wish to perform the following NFS mount action: mount titan:/usr /usr.

The in-kernel actions that proceed, assuming that all export and mount permissions are successful, are the following:

1.

A new vfs is created and is passed on to nfs_mount.

2.

nfs_mount fills in the new vfs structure with the vfs operations structure for NFS, and sets the v_vfsmountedhere of the vnode X2 to this new vfs.

3.

nfs_mount also creates a new vnode to serve as the root vnode of the Y file system as mounted on X. It stores this vnode in the v_data field of the new vfs structure.