Structure of the z/OS UNIX file system

If you follow the instructions for ServerPac and CBPDO installations, all z/OS elements and features that store into the z/OS® UNIX file system are installed into a consolidated file system, instead of having separate product-related file systems. Continue this consolidated approach as you install additional products on the platform. This method makes maintaining and cloning the file system easier, and it simplifies the MOUNT statements in the BPXPRMxx member.

Rule: You must maintain a separate file system for each of the following directories. When you are sharing a file system between systems, those four directories must have individual copies on each system. That is, each system should have their own copy of those four file systems mounted under those directories. You cannot share them between systems.

/etc, which contains customization data. Keeping the /etc file system in a file system separate from other file systems allows you to separate your customization data from IBM's service updates. It also makes migrating to another release easier. As described in Establishing an /etc file system for a new release, after you complete instructions for a ServerPac or CBPDO installation, you will have an /etc file system in its own file system.
/dev, which contains character special files that are used when logging into the shell environment and also during c89 processing. It is shipped empty. The necessary files are created when the system is IPLed, and on a per-demand basis.
/tmp, which contains temporary data that are used by products and applications. /tmp, is created empty, and temporary files are created dynamically by different elements and products. You have the option of mounting a temporary file system (TFS) on /tmp. For more information, see Managing the temporary file system (TFS).
/var, which contains dynamic data that is used internally by products and by elements and features of z/OS. Any files or directories that are needed are created when code is executed or customized. An example is caching data. In addition, you can be assured that IBM® products will only create files under /var when code is executed or customized.