General overview

z/OS UNIX System Services (z/OS UNIX) allows you to install virtual file system servers (VFS servers) and physical file systems (PFSs).

A VFS server makes requests for file system services on behalf of a client. A VFS server is similar to a POSIX program that reads and writes files, except that it uses the lower-level VFS callable services API instead of the POSIX C-language API.
An example of a VFS server is the Network File System.
A physical file system (PFS) controls access to data.
PFSs receive and act upon requests to read and write files that they control. The format of these requests is defined by the PFS interface.

PFSs include pipes, sockets, the Network File System client, and the following UNIX file systems: HFS, zFS, and TFS.

Another name for a PFS is an installable file system.

User-written programs use the POSIX API to issue file requests. VFS servers use the VFS callable services API to issue file requests. These requests are routed by the logical file system (LFS) to the appropriate PFS through the PFS interface. See Figure 1 for a view of this structure.

This information unit describes these two interfaces and discusses the things you need to know to write a VFS server or a PFS, or to port one to the z/OS UNIX environment. In order to do this, you should be a system programmer who is familiar with POSIX or UNIX.

Porting note

This information unit uses notes like this one to highlight certain points of the implementation that are particularly important to readers who are considering porting an existing UNIX-based program to z/OS UNIX.

z/OS UNIX supports the following types of files:

Regular files
Directories
Symbolic links
Character special files (for example, terminals)
Pipes (both FIFOs and unnamed)
Sockets

Restriction: Character special and unnamed pipe physical file systems cannot be implemented with this interface. Unnamed pipes and socket files cannot be exported by a VFS server.