A VFS server is a program that registers as a VFS server with z/OS UNIX by calling the v_reg() function. There is no special system definition required to become a VFS server.

A VFS server is a program that uses the VFS callable services API to access objects in the z/OS UNIX file hierarchy.

This is not to be confused with other types of servers. For example, consider a file transfer program that moves files between z/OS UNIX and a workstation. If this program uses the open(), read(), and write() functions to access the files, it is certainly a "file server", but this type of program it is not discussed in this topic. On the other hand, if this same program uses the v_get() and v_rdwr() functions, it is the type of server discussed here. Such a program could be written as a set of LU 6.2 transactions, independent of which interface is used to access the files.

The VFS callable services API is designed to meet the requirements of an NFS- or DFS-style server, but it is not limited to those applications. The main difference between the POSIX API and the VFS callable services API is that POSIX programs refer to files by path names and VFS servers refer to them by file identifiers (FIDs). VFS servers do their own path name resolution to convert a path name into a FID, and later use the FID to access the file. The FID is designed to be part of the NFS file handle that the Network File System returns to its clients. A file handle always refers to the same file. A path name, on the other hand, may refer to different files over time, because of rename, remove/re-create, or symbolic link changes.