The root file system and mountable file systems

Taken as a whole, the file system is the entire set of directories and files, consisting of all files shipped with the product and all those created by the systems programmer and users. The systems programmer (superuser) defines the root file system; subsequently, a user with mount authority can mount other mountable file systems on directories within the file hierarchy. (See the section on mount authority in z/OS UNIX System Services Planning.) Altogether, the root file system and mountable file systems comprise the file hierarchy used by shell users and applications.

After installation of z/OS®, the end user's logical view of the file system is as shown in Figure 1.

Figure 1. End user's logical view of the file system The file system can include symbolic links, as shown in this graphic.

System programmers need to know that the illustration of directories in Figure 1 is not a true representation of file systems. The file system, as installed through ServerPac or CBPDO, consists of /dev, /tmp, /var, and /etc symbolic links that point to the /dev, /tmp, /var, and /etc directories, as demonstrated in Figure 2.

Figure 2. Organization of the file system From the end user's viewpoint, all the subdirectories are under the main directory (the root).

Here SYSTEM is a data set that contains directories which are used as mount points, specifically for the /etc, /var, /tmp, and /dev file systems. IBM® requires that you mount /etc, /var, /dev, and /tmp in separate data sets.

The z/OS shells and utilities typically impose a line orientation on the byte-oriented files. A line is a stream of bytes terminated with a <newline> character. A line terminated by a <newline> character is sometimes referred to as a record. So, there is a single <newline> character between every pair of adjacent records. Text files use the <newline> character to delimit lines; binary files do not.

Several types of file systems can be mounted within the file hierarchy:

z/OS File System (zFS): zFS is the strategic file system for z/OS UNIX and continues to be enhanced to provide superior performance, reliability, and data integrity. zFS file systems can be mounted into the z/OS UNIX hierarchy along with other local (or remote) file system types (for example, HFS, TFS, AUTOMNT and NFS). Also, zFS can support access control lists (ACLs). For more information, see z/OS Distributed File Service zFS Administrationand z/OS Migration.
Hierarchical File System (HFS): In HFS, the entire file hierarchy is a collection of hierarchical file system data sets (HFS data sets). Each HFS data set is a mountable file system. DFSMS facilities are used to manage an HFS data set, and DFSMS Hierarchical Storage Manager (DFSMShsm) is used to back up and restore an HFS data set.
Network File System (NFS): Using NFS client on z/OS UNIX System Services, you can mount a file system, directory, or file from any system with an NFS server within your user directory. You can edit or browse the files. For more information, see Using the Network File System feature and z/OS Network File System Guide and Reference.
Distributed File System (DFS): DFS joins the local file systems of several file server machines, making the files equally available to all DFS client machines. DFS allows users to access and share files stored on a file server anywhere in the network, without having to consider the physical location of the file. For more information, see z/OS DFSMS Using Data Sets.
Temporary File System (TFS): The TFS is an in-memory physical file system that delivers high-speed I/O. To take advantage of that, the systems programmer (superuser) can mount a TFS over the /tmp directory so it can be used as a high-speed file system for temporary files. (Normally, the TFS is the file system that is mounted instead of the HFS if the kernel is started in minimum setup mode.)