VFS server – LFS control block structure

Files are contained within mounted file systems, and the collection of all the files in all the mounted file systems forms the z/OS UNIX file hierarchy.

The LFS structures for files and file systems are not directly addressable by a VFS server. Consequently, files and file systems are abstracted somewhat on the VFS callable services API.

A file is represented to a VFS server by a vnode token with the following characteristics:

A vnode token is similar to a POSIX file descriptor, in that it is the main input to all calls that refer to the file.
Vnode tokens are obtained most often from v_get() and v_lookup(), but also from v_rpn(), v_create(), and v_mkdir()
Vnode tokens are not inherited across fork().
Vnode tokens are released with v_rel(). All vnode tokens that are obtained must eventually be released. After v_rel() is called, any subsequent call with the same vnode token fails.
A single vnode token may be cached by the VFS server and shared among many end users. A single vnode token can be used by several tasks at the same time, but v_rel() is mutually exclusive with all other operations.
Many different vnode tokens can be obtained for the same file.
A vnode token that has not been released is always valid for a call in the sense that the VFS server program will not abnormally end from using it.
Files that are deleted are still accessible with existing vnode tokens. This is the same behavior that is expected for POSIX file descriptors that have not been closed. If the underlying real file system is unmounted with IMMEDIATE or FORCE, however, calls that are made with vnode tokens for files in that file system fail with an error code.

A file system is represented to a VFS server by a VFS token with the following characteristics:

The VFS token represents a virtual file system (VFS). With NFS, for example, this corresponds to a client mount. The directory that is mounted becomes the root of this VFS.
A VFS is a subset of some real mounted file system. VFS servers do not refer to the mounted file system directly.
VFS tokens are obtained from v_rpn(), and they are never released.
VFS tokens are used only with the v_get() function, which converts a file ID within a given VFS into a vnode token.
All VFS tokens for VFSs that are contained within the same real mounted file system are the same.
VFS tokens remain valid for as long as the underlying real file system is mounted.
After the underlying file system is unmounted, v_get() with the prior VFS token fails with an error code. This remains true even if the real file system is remounted.