Z/OS is a 64-bit operating system (OS) developed by IBM for its family of z/Architecture enterprise mainframe computers, including the zEnterprise 196 and zEnterprise 114.

Z/OS is described as an extremely scalable and secure high-performance operating system based on the 64-bit z/Architecture. Like its predecessor, OS/390, z/OS lays claim to being highly reliable for running mission-critical applications. The operating system supports web- and Java-based applications.

How is z/OS used

Mainframe computers have been a staple of the enterprise data center for decades, focusing enterprise computing power into a single powerful, cohesive, resilient and secure system. Such a hardware platform requires an operating system capable of operating Multi-tenant mission-critical workloads. Z/OS is IBM's latest operating system that can manage many multi-tenant mainframe applications, each running in protected memory spaces and offering varied performance goals.

Mainframes with z/OS are typically used to run large, complex, mission-critical workloads for large enterprise organizations. Generally mainframes are well-suited for tasks involving large-scale transaction processing. They can handle thousands of simultaneous users and many applications accessing computing resources, manage databases in the terabyte size range and run busy high-bandwidth network communication. Such workloads often include real-time order entry and processing, financial transactions, payroll, inventory control, manufacturing and production. For example, a busy web store might use a mainframe host on the back end for security, scalability and reliability, while a bank might run its fleet of ATMs from a mainframe for that same reason.

Z/OS features

As a mainframe operating system, z/OS provides a wide array of critical features and functionality.

Z/OS starts with a Base Control Program (BCP) that provides core OS services, including the I/O configuration program (IOCP), Workload Manager (WLM), system management facilities (SMF), z/OS Unix System Services (z/OS Unix) kernel, the program management binder (FMID HPM7790), IBM Health Checker for z/OS, support for the Unicode Standard (FMID HUN7790), z/OS XML System Services (z/OS XML), Capacity Provisioning (FMID HPV7790) and System Restructured Extended Executor (REXX) for z/OS Base.

Storage and data features include Bulk Data Transfer (BDT) services that other BDT tools need to transfer data between computer systems. A Distributed File Service runs the Unix high-performance zSeries File System (zFS) in addition to the hierarchical file system (HFS) to manage files and directories and provides server message block (SMB) file/print server support. Network File System (NFS) support provides a file server to workstations, personal computers and other systems in a TCP/IP network. Modules such as DFSMSdfp offer storage, data, program and device management functions. DFSMSdss copies and moves data for backup and recovery while reducing disk fragmentation. DFSMShsm provides automated direct access storage device (DASD) management to help mitigate data loss.

Language features include an alternate library for REXX, which allows users to run compiled REXX programs. A High Level Assembler (HLASM) Toolkit offers developer tools to improve application development, debugging and recovery. An Interactive System Productivity Facility (ISPF) provides tools for host-based software development, including a Dialog Manager (DM) for visual interface components, a Program Development Facility (PDF) to assist developers in editing and compiling, a Software Configuration and Library Manager (SCLM) to manage all of the software components of an application and a client-server component. The Language Environment provides a runtime environment for programs generated with C, C++, COBOL, Fortran and PL/I languages. The Metal C Runtime Library offers LPA-resident C functions that IT can call from a C program created with the z/OS XL C compiler.

Document management features include BookManager Build, which creates documents for any BookManager products, such as BookManager Read to display, search, and manage online documents or Library Server to convert BookManager and InfoCenter documents to HTML for web browsers. The GDDM group of features provides presentation capabilities. IBM HTTP Server is the web server for z/OS designed to serve critical e-business applications. Infoprint Server allows administrators to print files on z/OS printers from any networked workstation.

Systems management features include support for the Common Information Model (CIM), which allows system administrators to create applications to gauge system resources in a heterogeneous network. First Failure Support Technology (FFST) provides immediate alerting and data capture for software failure events. A Hardware Configuration Definition (HCD) defines the operating system and processor configurations for a system. The IBM Tivoli Directory Server for z/OS (IBM TDS) offers an LDAP server, an LDAP client and LDAP client utilities. The Open Systems Adapter/Support Facility (OSA/SF) monitors and controls network connectivity local area networks (LANs) and wide area networks (WANs). A Resource Measurement Facility (RMF) gathers data about z/OS resource use and performance. The System Display and Search Facility (SDSF) allows administrators to monitor, manage and control z/OS systems. An SMP/E tool can install and maintain software, and help manage the installed software.

Communications features include Communications Server to support secure TCP/IP, SNA and Unix networking, allowing administrators to connect systems, devices and applications. ZEnterprise Data Compression (zEDC) for z/OS supports direct data compression using zEC12 and zBC12 zEDC Express adapters.

Security features include powerful Cryptographic Services to protect and sign data, as well as manage cryptographic keys as long as 56 bits. Longer keys are possible with optional z/OS Security Level 3 features. Integrated Security Services provide Enterprise Identity Mapping (EIM) to map a user's identity on one system to the user's identity on another system, a Network Authentication Service, and supports Open Cryptographic Enhanced Plug-ins (OCEP).

Z/OS provides more core and many other optional features, and the feature set may change and expand over time.

Z/OS major releases

Z/OS was announced in October 2000 and first appeared as version 1 revision 1 (V1R1) on March 30, 2001. The current version, V2R2, became generally available in September 2015 and the next version, V2R3 is expected to appear in late September 2017. However, IBM regularly updates z/OS in the intervening 16 plus years:

z/OS releases
Major z/OS releases

Z/OS pros and cons

Z/OS has established a long pedigree of reliable operation -- it's a proven, tried-and-true mainframe platform with close hardware support. The careful attention to workload management and scheduling and security can be vital attributes for busy enterprises running critical applications.

But z/OS, like the z/Architecture mainframes it runs on, can be an expensive investment that can include a proprietary monthly license charge, pricing based on usage and reduced pricing options for some applications. The choice of available operating systems can also be a serious issue. Z/Architecture systems like the zEC12, zBC12, z114 and z196 will support IBM's native operating systems such as z/OS, z/VM, z/VSE, and z/TPF. Fortunately, cross-platform OS compatibility is improving, and other operating systems can be used on some z/Architecture systems, including Linux on z Systems, such as Red Hat Enterprise Linux 6 and SUSE Linux Enterprise Server 11. IBM has also Microsoft Windows Server support on x86 processor-based blades that integrate with the IBM zEnterprise BladeCenter Extension (zBX).

Still, IBM native operating systems such as z/OS are closed source, and users completely rely on IBM to maintain and update the code. Users may be forced into updates and associated costs as support expires on aging OS versions. Linux variations are open source, but this carries risks of roadmap issues and code exposure, which large enterprises may not desire.

Z/OS supports a wide range of programming platforms and tools. Low-level programs can be written in assembler language. Common Business-Oriented Language (COBOL) is for business applications often associated with commercial data processing. Programming Language/I (PL/I) offers a general-purpose, full-featured and high-level programming language. REXX is a procedural language that allows programmers to construct programs and algorithms in structural manner. More recent language support includes programs written in C/C++, Java applications.

Z/OS, Linux, and Unix

Generally, using Linux instead of z/OS can make it easier to run current business applications on a mainframe, while z/OS provides a solid and predictable platform for decades-old mainframe applications. It's also easier to find and retain developers and other IT professionals with a solid background in Linux support. But z/OS is typically preferred over Linux for more traditional considerations such as running COBOL or native z/OS applications. Remember that Linux will use a virtual environment through z/VM so Linux and its applications may not run quite as efficiently as similar applications designed for native z/OS. Fortunately, the choice between z/OS and Linux is not an all-or-nothing proposition, and organizations can run both operating systems together using z/VM.

Administrators may also need to weigh the use of z/OS versus Unix such as Solaris, AIX and HP-UX. Generally, smaller platforms using commercial software applications would possibly benefit from Unix for its lower costs and larger support base. When the applications are being developed and maintained in-house, Unix may offer some appeal, but demanding, transaction-intensive mainframe applications will likely do better sticking with z/OS. Z/OS is also a preferred platform for security and reliability -- particularly for mission-critical applications. Unix can frequently run alongside z/OS using z/OS Unix System Services (USS).

This was last updated in October 2017

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