Your installation's operating system environment consists of several different types of code and data– some supplied by IBM®, some by other vendors, or some by your company's programmers– all of which combine to make your z/OS® system unique. Once these software products and applications have been organized and stored, system programmers spend much of their time planning when to install updates or new software products, how to test them, and when to move them into production systems without adversely affecting workloads.

Figure 1 illustrates the different types of software code and data that exist in a system, and how they are usually organized in system storage.

Figure 1. Typical organization of software

The z/OS software– as supplied by IBM– is usually installed on a series of disk volumes known as the system residence volumes (SYSRES). Much of the flexibility of z/OS is built on these SYSRES sets. They make it possible to apply maintenance to a new set that is cloned from the production set while the current set is running production work. A short outage can then be taken to IPL from the new set--and the maintenance has been implemented! Also, the change can be backed out by IPLing from the old set.

Fixes to z/OS are managed with a product called System Modification Program/Extended (SMP/E). Indirect cataloging using system symbols is used so that a particular library is cataloged as being on, for example, SYSRES volume 2, and the name of that volume is resolved by the system at IPL time from the system symbols.

Other IBM software (such as CICS® and DB2®), and non-IBM or third-party vendor software products are usually installed on another group of volumes, rather than on the SYSRES volumes. The SYSRES sets are usually managed as one entity by SMP/E, so their content is usually limited to z/OS software. The non-z/OS software is installed on as many volumes as are required, and thus can be managed separately.

Another group of volumes is reserved for customization data, which refers to data such as the z/OS system libraries (SYS1.PARMLIB and SYS1.PROCLIB, for example); the master catalog; the I/O definition file (IODF); page data sets; job entry subsystem (JES) spools; the /etc directory; and other items that are essential to the running of the z/OS system. It is also where SMP/E data is stored to manage the software.

These data sets are not always located on separate DASD volumes from IBM-supplied z/OS software; some installations place the PARMLIB and PROCLIB on the first SYSRES pack, others place them on the master catalog pack or elsewhere. This is a matter of choice and is dependent on how the SYSRES volumes are managed. Each installation will have a preferred method.

On many systems, some of the IBM-supplied defaults are not appropriate, so they need to be modified. User exits and user modifications (usermods) are made to IBM code so that it will behave as the installation requires. The modifications are usually managed using SMP/E.

Finally, another set of volumes contains production, test, and user data; this set is usually the largest pool of disk volumes. This set of volumes is not part of the system libraries, but is presented here for completeness. It is often split into pools and managed by System Managed Storage (SMS), which can target data to appropriately managed volumes. For example, production data can be placed on volumes that are backed up daily, whereas user data may only be captured weekly and may be migrated to tape after a short period of inactivity to free up the disk volumes for further data.