Enterprise IT Consultant Views on Technologies and Trends

Feb 14 2011   1:50AM GMT

Understanding Mainframe – Overview for non-mainframe personnel

Sasirekha R Profile: Sasirekha R

Mainframe Overview with focus on Cost Savings

Recently one of our Unix experts – focusing on IT optimization – came to me to get an understanding of Mainframe and like most IT people who have not worked in Mainframe had quite a few obsolete notions. So I created a set of documents on Mainframe overview, on zOS (operating system) and CICS (the transaction processor) and sharing the same here.

Mainframe is a self-contained processing center, powerful enough to process the largest and most diverse workloads in one secure “footprint.” At the same time, Mainframe is also just as effective when implemented as the primary server in a corporation’s distributed server farm.

There are different classes of mainframe to meet diverse needs of customers – Business Class and Enterprise Class.

Security, scalability, and reliability are the key criterions that differentiate the mainframe. Businesses today rely on the mainframe to:

  • Perform large-scale transaction processing (thousands of transactions per second)
  • Support thousands of users and application programs concurrently accessing numerous resources
  • Manage terabytes of information in databases
  • Handle large-bandwidth communication

Typically, the mainframe shares space with many other hardware devices: external storage devices, hardware network routers, channel controllers, and automated tape library “robots,” etc. Unlike in the past, now, the mainframe is physically no larger than many of these devices and generally does not stand out from the crowd of peripheral devices (earlier Mainframe used to have rooms for themselves but now they are just part of a data center).

Mainframe interfaces today look much the same as those for personal computers or UNIX systems. A business application is accessed through a Web browser, with the mainframe in the background.

It is now possible to run a mainframe operating system on a PC that emulates a mainframe. Such emulators are useful for developing and testing business applications before moving them to a mainframe production system (can be useful in cost savings).

Most mainframe workloads fall into one of two categories: batch processing or online transaction processing, which includes Web-based applications. Today’s mainframe can run standard batch processing such as COBOL as well as batch UNIX and batch Java programs.

A mainframe can be the central data repository, or hub, in a corporation’s data processing center. For example, centralizing the data in a single mainframe repository can save customers from having to manage updates to more than one copy of their business data, which increases the likelihood that the data is current.

Mainframe – Hardware

Mainframe hardware consists of processors and a multitude of peripheral devices such as disk drives (called direct access storage devices or DASD), magnetic tape drives, and various types of user consoles;

 o The term Box may refer to the entire machine or model; it is an expression used due to its shape. Mainframe systems today are much smaller than earlier systems-about the size of a large refrigerator. The mainframe’s power consumption today is 0.91 watts per MIPS and is expected to decrease with future models.

o The abbreviation CEC, pronounced keck, is for the Central Electronic Complex that houses the central processing units (CPUs).

o Central processor complex or CPC refers to the centralized processing hub that contains the processors, memory, control circuits, and interfaces for channels.

o All the processors (S/390 or z/Architecture) present in the CPU are referred to as processing units (PUs). The PUs are characterized as CPs (for normal work), Integrated Facility for Linux (IFL), Integrated Coupling Facility (ICF) for Parallel Sysplex configurations and so forth.

o A channel provides an independent data and control path between I/O devices and memory. Today, the largest mainframe can have over 1000 channels. A channel can be considered a high-speed data bus. Todays mainframe use ESCON (Enterprise Systems CONnection) and FICON (FIber CONnection) channels.

o Channels connect to control units. A control unit contains logic to work with a particular type of I/O device – printers, tape drives etc. Today’s channel paths are dynamically attached to control units as the workload demands – providing a form of virtualizing access to devices.

o Control units connect to devices, such as disk drives, tape drives, communication interfaces, and so forth.

o Sharing of I/O devices is common in all mainframe installations. A technique used to access a single disk drive by multiple systems is called multiple allegiance. Multiple paths to a device allows for effective disk sharing (across multiple servers) which in turn can provide improved performance and availability.

The IBM mainframe can be partitioned into separate logical computing systems. System resources (memory, processors, I/O channels) can be divided or shared among many such independent logical partitions (LPARs).

For many years there was a limit of 15 LPARs in a mainframe; today’s machines can be configured with up to 60 logical partitions. Practical limitations of memory size, I/O availability, and available processing power usually limit the number of LPARs to less than these maximums. Logical partitions are, in practice, equivalent to separate mainframes. Right sizing is the key when it comes to partitions. It can be used to control usage of resources, improved security, availability etc.


z/OS – widely used mainframe operating system – system is a share-everything runtime environment that provides for resource sharing through its heritage of virtualization technology. z/OS gets work done by dividing it into pieces and giving portions of the job to various system components and subsystems that function interdependently.

A z/OS system usually contains additional, priced products that are needed to create a practical working system. IBM refers to its own add-on products as IBM licensed programs – and comes at a cost. Also independent software vendors (ISVs) offer a large number of products with varying but similar functionality, such as security managers and database managers. The typical products include:

  • Security System (Resource Access Control Facility – RACF from IBM)
  • Compilers (z/OS includes an assembler and a C compiler. Other compilers – COBOL, PL/1 etc are separate products).
  • Relational Database (DB2 from IBM)
  • Transaction processing facility (CICS, IMS, WAS from IBM)
  • Sort (DFSORT from IBM)
  • Utility Programs

Even the System Display and Search Facility (SDSF) program that people use extensively to view output from batch jobs is a licensed program (looking at these licensed programs to see if they are really necessary – and also that there is no other ISV product with similar functionality available in the installation provides another opportunity for cost saving).

Besides z/OS, four other operating systems dominate mainframe usage: z/VM, z/VSE, Linux for zSeries, and z/TPF. The use of z/OS, z/VM, and Linux on the same mainframe is common.

It is also important to note that if there are multiple versions of the same software in an installation, the license costs are actually multiplied.

Mainframe provides Specialty engines -zAAP as a specialized Java execution environment, IFL for Linux – which enables off-loading specific work to separate processors.  As attractive prices are offered for such processors, the overall total cost of ownership reduces by appropriate use of these processors. This also enables the general CPUs to continue processing standard workload increasing the overall ability to complete more batch jobs or transactions.

Consolidation of Mainframe

Data center consolidation initiatives have resulted in several smaller mainframes being replaced with fewer but larger systems.

Software license costs for mainframes have become a dominant factor in the growth and direction of the mainframe industry as mainframe software (from many vendors) can be expensive, often costing more than the mainframe hardware.  Though Software license costs are often linked to the power of the system, yet the pricing curves favor a small number of large machines – replacing multiple software licenses for smaller machines with one or two licenses for larger machines is cost effective (While consolidating mainframes, the licenses for software (with 3rd party vendors) may have to be re-negotiated for cost savings ).

The relative processing power needed to run a traditional mainframe application (a batch job written in COBOL, for example) is far less than the power needed for a new application (with a GUI interface, written in C and Java). New powerful mainframes might need only 1% of their power to run an older application, but the application vendor often sets a price based on the total power of the machine, even for older applications.

As an aid to consolidation, the mainframe offers software virtualization, through z/VM. z/VM’s extreme virtualization capabilities and Linux on Mainframes make it possible to virtualize thousands of distributed servers on a single server. Consolidating distributed servers to mainframe can directly translate into significant monetary savings (IBM’s has conducted a very large consolidation project named Project Big Green to consolidate approximately 3,900 distributed servers into roughly 30 mainframes, using z/VM and Linux on System z. It achieved reductions of over 80% in the use of space and energy.).

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  • Sasirekha R
    IBM has published an excellent Redbook on the New Mainframe - relevant to both mainframe and non-mainframe personnel. http://www.redbooks.ibm.com/redbooks/pdfs/sg246366.pdf
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