Glossary | Linux on the Mainframe

This glossary defines technical terms and abbreviations used in this book. If you do not find the term you are looking for, the Web is likely to produce a quick result. One place to check is the IBM Dictionary of Computing, located at: http://www.ibm.com/networking/nsg/nsgmain.htm.

ACL.: Access control list. (1) In computer security, a collection of all access rights for one object. (2) In computer security, a list associated with an object that identifies all the subjects that can access the object and their access rights; for example, a list associated with a file that identifies users who can access the file and identifies their access rights to that file.

address space.: An address space is a range of contiguous virtual storage addresses, unique for each user, that the operating system creates. A small portion of the address space has system data and programs mapped into it, to ensure fast execution of system requests.

API.: Application programming interface. A defined software interface that enables programs to communicate with each other. An API is the set of programming language constructs or statements that can be coded in a program to obtain the specific functions and services provided by an underlying operating system or another program.

application.: A collection of software components used to perform specific types of user-oriented work on a computer.

ASCII.: American Standard Code for Information Interchange. ASCII is widely used outside of the mainframe. It relates to a coding scheme that is used to hold one character of information in one byte (8 bits). For the ASCII coding sheet, see http://www.ibm.com/servers/eserver/iseries/software/globalization/pdf/cp00038z.pdf.

ASP.: Application service provider. A company that offers application software to clients on a centrally located (and centrally-managed) host. Businesses gain the benefit of running sophisticated applications while avoiding the cost of deploying the required hardware and software in-house.

autonomic computing.: Autonomic computing is a technology that can manage and improve its own operation with minimal human intervention. For further information, see http://www.ibm.com/autonomic/index.shtml

back-end.: A part of an application that is the least visible to its user. Typically, the back-end is that part of the application logic that contains the database and transaction manager.

BSD.: Berkeley Software Distribution. Pertains to any of the series of UNIX specifications or implementations distributed by the University of California at Berkeley. The abbreviation BSD is usually followed by a number to specify the particular version of UNIX that was distributed (for example, BSD 4.3). Many vendors use BSD specifications as standards for their UNIX products.

CCW.: Channel command word. A doubleword at the location in main storage specified by the channel address word. One or more CCWs make up the channel program that directs data channel operations.

channel.: Communication link (parallel, ESCON) between system and control unit.

channel-to-channel adapter (CTCA).: An adapter that cross-connects a pair of channels between systems to allow for point-to-point communications between two systems.

CHPID.: Channel path identifier. In a channel subsystem, a value assigned to each installed channel path of the system that uniquely identifies that path to the system.

CIM.: Common information model. An implementation-neutral, object-oriented schema for describing system management information. The Distributed Management Task Force (DMTF) develops and maintains CIM specifications.

CISC.: Complex instruction set computer. Such computers have a large number of instructions, typically 100 or more. Instructions are provided for most common programming tasks, for example, there would be one instruction to move hundreds or thousands of bytes, saving the need for a program loop to move the data.

CMOS.: Complementary metal-oxide semi-conductor. CMOS is the semiconductor technology used in the transistors that are manufactured into most of today's computer microchips.

CMS.: Conversational Monitor System. A Virtual Machine operating system that is used for general interactive time-sharing, problem solving, and program development capabilities, and in recent years operates only under control of the VM Control Program.

COBOL.: Common Business Oriented Language. COBOL was the first widely used high-level programming language for business applications. Many payroll, accounting, and other business application programs written in COBOL over the past 35 years are still in use, and it is possible that there are more existing lines of programming code in COBOL than in any other programming language.

connector.: A connector provides access to resources of another system image. For example, an application server running on Linux may connect to data of a z/OS or VSE/ESA system distribution.

consolidation.: See [server consolidation]

control unit.: I/O controller attached to a channel supporting up to 255 devices.

CP.: Control Program. The base component of z/VM that manages the resources of a single computer so that multiple computing systems appear to exist. Each of these apparent systems, or Virtual Machines, is the functional equivalent of an IBM System/370, 370-XA, ESA computer, or z/Architecture computer.; See also [Virtual Machine]

CPU.: Central processing unit. The part of a computer that includes the circuits that control the interpretation and execution of instructions. Traditionally, the complete processing unit was often regarded as the CPU, whereas today the CPU is often a microchip.

CPU utilization.: A measure of performance. This is a measure of how busy the CPU is at any given period of time, for example, 24 hours.

CTCA.: See [channel-to-channel adapter]

CTSS.: Compatible Time Sharing System. This IBM 7094 time-sharing operating system was created at MIT Project MAC and first demonstrated in 1961. It was used as the programming and debugging tool during initial Multics programming and bring up by Project MAC, General Electric CISL, and Bell Telephone Lab personnel. In some sense the intellectual offspring of CTSS, Multics was conceived as the next step in the future of multiple-access computing.

DASD.: Direct Access Storage Device. DASD is a general term for magnetic disk storage devices that has historically been used in the mainframe and minicomputer (mid-range computer) environments. The "direct access" means that all data can be accessed directly in about the same amount of time rather than having to progress sequentially through the data.

datamart.: A subset of a data warehouse that contains data tailored and optimized for the specific reporting needs of a department or team. A datamart can be a subset of a warehouse for an entire organization, such as data contained in online analytical processing (OLAP) tools.

device.: A mechanical, electrical, or electronic contrivance with a specific purpose.; See also [DASD]

device number.: In a channel subsystem, the four hexadecimal digits that uniquely identify an I/O device.

DFSMS.: Data Facility Storage Management Subsystem. An IBM feature for managing files in Shared File System (SFS) file pools, helping to move CMS minidisks between like and unlike direct access storage devices (DASD) quickly and efficiently, and providing command and Callable Services Library (CSL) interface to removable media library support.

directory.: (1) In a hierarchical file system, a container for files or other directories. (2) In VM, a Control Program (CP) disk file that defines each Virtual Machine's typical configuration: the user ID, password, regular and maximum allowable virtual storage, CP command privilege class or classes allowed, dispatching priority, logical editing symbols to be used, account number, and CP options desired.

DirMaint.: Directory maintenance. A CMS application that helps manage an installation's VM directory. Directory management is simplified by the DirMaint command interface and automated facilities. DirMaint directory statement-like commands are used to initiate directory transactions. DirMaint error checking ensures that only valid changes are made to the directory, and that only authorized personnel are able to make the requested changes. Any transaction requiring the allocation or deallocation of minidisk extents can be handled automatically. All user-initiated transactions can be password-controlled and can be recorded for auditing purposes.

disaster recovery.: The process used to recover from an outage in the IT environment due to the simultaneous failure of multiple key components that preclude simply restarting the failed environment. However, such a simultaneous failure is unusual. The effects can be disastrous in that the length for recovery is long and the outage impact could hurt the business. Such multiple simultaneous outages usually are related to an event that is called a "disaster," such as a fire, earthquake, or explosion.

distribution.: A Linux distribution provides a collection of application, middleware, tools, kernel, device drivers, and libraries. Distributors, such as Red Hat and SuSE, ensure that key packages of a distribution are well-tested. They also provide installation and configuration capabilities as well as maintenance and support.

DMTF.: Distributed Management Task Force. An alliance of computer vendors that was convened to define streamlined management of the diverse operating systems commonly found in an enterprise. For further information, see http://www.dmtf.org/

EBCDIC.: Extended Binary-Coded-Decimal Interchange Code. EBCDIC is a binary code for alphabetic and numeric characters that IBM developed for its larger operating systems. It is the code for text files that is used in IBM's z/OS operating system for its zSeries servers and that corporations use for their legacy applications and databases. In an EBCDIC file, each alphabetic or numeric character is represented with an 8-bit binary number (a string of eight 0s or 1s). 256 possible characters (letters of the alphabet, numerals, and special characters) are defined.

ECC.: Error correction code or error checking and correcting. ECC allows data that is being read or transmitted to be checked for errors and, when necessary, corrected on the fly. It differs from parity-checking in that errors are not only detected but are also corrected.

ESCON.: Enterprise Systems Connection. A set of IBM products and services that provide a dynamically connected environment within an enterprise based upon fiber connections.

ESS.: IBM TotalStorage Enterprise Storage Server. An advanced, SAN-ready disk storage system that provides high performance, scalability, and universal access across all major server platforms, and which maximizes data sharing across the enterprise.

external throughput rate (ETR).: Work completed per second. For example, a Web server serving 300 pages per second can be said to have an ETR of 300 pages per second.

FCP.: Fibre Channel Protocol. A standardized protocol recently implemented by IBM on its zSeries mainframe. There is a family of these protocols used for transporting SCSI over a fiber channel, instead of over copper. The zSeries FCP channel provides unique value in Linux environments. For further information on FCP, see http://www.t10.org/scsi-3.htm.

FICON.: Fiber Connectivity. FICON is a high-speed input/output (I/O) interface for mainframe computer connections to storage devices. As part of IBM's zSeries server, FICON channels increase I/O capacity through the combination of a new architecture and faster physical link rates to make them up to eight times as efficient as ESCON (Enterprise System Connection), IBM's previous fiber-optic channel standard.

FORTRAN.: FORTRAN (FORmula TRANslation) is a third-generation programming language that was designed for use by engineers, mathematicians, and other users and creators of scientific algorithms.

front-end.: A part of an application that is the closest to the user. Typically, it contains that part of the application logic that manages the application's presentation to the user.

GDPS.: Geographically Dispersed Parallel Sysplex. An IBM solution for disaster recovery and continuous availability for a zSeries multi-site enterprise. This solution mirrors critical data and efficiently balances workload between the sites. GDPS solution also uses automation and Parallel Sysplex technology to help manage multi-site databases, processors, network resources, and storage subsystem mirroring.

GPL.: GNU General Public License. A software license allowing the licensed program to be passed on, on condition that the source text always remains available. Equally permissible and desirable is the fact that you can make your own improvements to the software and republish it. Linux itself is also under the GPL. For further information, see http://www.gnu.org/copyleft/gpl.html.

Guest LAN.: z/VM 4.2 introduced Guest LAN support, which is implemented in the z/VM Control Program (CP). With the Guest LAN interfaces, you can define a virtual LAN. z/VM guests may communicate via TCP/IP and virtual network interfaces with other guests connected to such a virtual (Guest) LAN.

HA.: High availability. Refers to a system or component that is operational for a desirably long length of time. Availability can be measured relative to "100% operational" or "never failing." A widely-held but difficult-to-achieve standard of availability for a system or product is known as "five 9s" (99.999 percent) availability.

high-availability clustering.: A high-availability (HA) cluster is a group of interconnected servers, redundantly configured, that behave as an aggregate to serve a particular workload. The cluster is configured with hardware and software so that the failure of one or a few servers in the cluster does not impact the availability of the application from the users' perspective.

horizontal growth.: Growing capability by adding a complete new instance of a resource. Contrasted with vertical growth, which is growing by increasing the capacity of the existing resources.

hypervisor.: The licensed internal code that was developed to support the Logical Partitioning (LPAR) of an SMP system. In z/VM, it is the part of the Control Program (CP) responsible for managing the virtual environment.

IFL.: Integrated Facility for Linux. An IFL is a feature of the G5, G5 and zSeries family of processors. These features are only available within a special LPAR defined to run in Linux-only mode.

internal throughput rate (ITR).: A performance measurement. Units of work completed per unit of processor busy time. For example, if a Web server serves 30 pages per second and thereby shows CPU as 50% busy, then its ITR is 60 pages per second. ITR only measures the throughput from within the processor. Compare with external throughput rate, which measures the end users' perspective of throughput.

I/O Configuration Data Set (IOCDS).: The file on the Support Element that contains an I/O and LPAR configuration definition built by the I/O configuration program (IOCP). This file is loaded at Power on Reset.

IRQ.: Interrupt Request Line. An IRQ is a hardware line used in a PC by (ISA bus) devices such as keyboards, modems, sound cards, and so on, to send interrupt signals to the processor to tell it that the device is ready to send or accept data.

ISP.: Internet Service Provider. An organization that provides access to the Internet for a fee.

Linux.: Linux is a UNIX-like operating system that was designed to provide personal computer users with a free or very low-cost operating system comparable to traditional and usually more expensive UNIX systems. Linux's kernel (the central part of the operating system) was developed by Linus Torvalds at the University of Helsinki in Finland. To complete the operating system, Torvalds and other team members made use of system components developed by members of the Free Software Foundation for the GNU project. Linux is distributed commercially by a number of companies.

Load balancing cluster.: A specific type of cluster that involves a front-end dispatcher which distributes the incoming application work to the servers within a cluster, attempting to assure that each server receives its share of the incoming work.

LPAR.: Logical PARtition. LPAR is the division of a computer's processors, memory, and storage into multiple sets of resources so that each set of resources can be operated independently with its own operating system instance and applications.

MAC.: Mandatory Access Control. In computer security, a means of imposing mandatory controls when restricting access to objects, based on the sensitivity, as represented by a sensitivity label, of the information contained in the objects and the formal authorization or security clearance of subjects to access information of that sensitivity, and enforced by the trusted computing base. A user only gets access to a controlled resource providing he/she passes the mandatory control test.

middle-tier.: A middle-tier "lives" between the front-end (tier) and the back-end (tier). Middle-tiers usually implement the piece of the application that collects the data (input) from the end user, does some initial processing, and passes that on to the back-end (and vice versa with the results). For example, a Web server is a typical middle-tier server.

middleware.: A vague term that refers to the software between an application program and the lower-level operating system functions.

minidisk.: (1) A z/VM construct that is a logical subdivision (or all) of a direct access storage device that has its own virtual device number, consecutive virtual cylinders (starting with virtual cylinder 0), and a Volume Table of Contents (VTOC) or disk label identifier. (2) A virtual disk in storage.

MIPS.: A measure of computer processing performance that is equal to one Million Instructions Per Second.

MTBF.: Mean time between failures. For a stated period in the life of a functional unit, the mean value of the lengths of time between consecutive failures under stated conditions.

multiple preferred guests (MPF).: A facility that supports up to six preferred Virtual Machines when the Processor Resource/Systems Manager (PR/SM) feature is installed in the real machine. This facility allows z/VM to provide special treatment for these guests, thus allowing nearly-native performance for those guests.

MVS.: Multiple Virtual Storage. MVS was IBM's first mainframe operating system to provide separate virtual address spaces for each of its users. Over time, it was succeeded by other operating systems like OS/390 and currently z/OS.

OCO.: Object code only. The format that IBM uses to ship many of its priced products. OCO refers to the fact that no source code is made available. In addition, the product license permits you to execute only the binary implementation and gives you no rights to the original product source code.

OS/MFT.: Operating System/Multi-programming with a Fixed number of Tasks. OS/MFT was one of the two heavyweight real-storage operating systems on the original S/360 machines (OS/MVT was the other one).

OS/MVT.: Operating System/Multi-programming with a Variable number of Tasks. OS/MVT was one of the two heavyweight real-storage operating systems on the original S/360 machines (OS/MFT was the other one). z/OS was derived from OS/MVT.

PKI.: Private Key Infrastructure. PKI is used with cryptography, and is an asymmetric key system consisting of a public key (which is sent to clients) and a private key (which stays local on the server). PKI differs from a symmetric key system in which both the client and server use the same key for encryption and decryption.

program status word (PSW).: Defined by the architecture as the location containing the state indicators that reflect the execution of the current program, as well as the address of the instruction currently being executed. The PSW is implemented as a register in the hardware.

PROP.: Programmable Operator. This CMS facility enables automatic filtering and routing of messages from a specified Virtual Machine's system console to another Virtual Machine. It also permits installation-defined actions to be automatically performed.

Processor Resource/Systems Manager (PR/SM) feature.: Provides the logical partitioning (LPAR) capability of the real machine such that multiple system-control programs can simultaneously share the use of the processors in a single machine. It also provides support for the multiple preferred guest facility of z/VM.; See also [multiple preferred guests]

quota.: The maximum amount of a resource that a resource consumer is allowed to use. Frequently this concept is used with respect to disk or file space. In UNIX and Linux systems, there is a quota command that is used to implement such a resource-capping process for file systems.

RACF.: Resource Access Control Facility. An IBM licensed program that provides access control by identifying users to the system; verifying users of the system; authorizing access to protected resources; logging detected, unauthorized attempts to enter the system; and logging detected accesses to protected resources. RACF is included in OS/390 Security Server and is also available as a separate program for the z/VM environment.

RAID.: Redundant Array of Independent Disks. RAID is a way of configuring redundant disks in various types of arrays so that the data can be retrieved, even if one disk in the array were to fail. There are many types of RAID configuration. The two most common configurations are: the mirroring approach of RAID1 (where each piece of data is written to two disks in the array) and the striping approach of RAID5 (where a block of data is striped across multiple disks, together with an error correction code that allows the data to be reconstructed if any disk fails). For further information, see http://www.acnc.com/04_01_00.html.

RAMAC.: RAID Architecture with Multi-level Adaptive Cache. RAMAC is an older IBM disk product that attaches to the mainframe.

RAS.: Reliability, availability, serviceability.

Response time.: A measure of the time taken to perform a certain task, such as running a specific job. However, another understanding of "response time" is the elapsed time from the point a user hits the Send key, until the point-in-time that the first byte of data comes back. Response time is related to ETR.

REXX.: Restructured Extended Executor. A general-purpose, interpretive, procedural language for end-user personal programming, designed for ease-of-use by both casual general users and computer professionals. It is also useful for application macros. REXX includes the capability of issuing commands to the underlying operating system from these macros and procedures. REXX is the automation scripting language used in z/VM.

RISC.: Reduced Instruction-Set Computer. A computer that uses a small, simplified set of frequently-used instructions for rapid execution.

RSA.: Rivest-Shamir-Adelman. The RSA algorithm is one of the most commonly used encryption and authentication algorithms today. The algorithm involves multiplying two large prime numbers and through additional operations deriving a set of two numbers that constitutes the public key and the private key. Both the public and private keys are needed for encryption and decryption, but only the owner of a private key ever needs to know the contents of the private key. Using the RSA system, the private key never needs to be sent across the Internet.

SAN.: Storage Area Network. This concept puts sets of disk storage on an area network and allows this network of disks to be used by multiple servers. By aggregating storage, the SAN becomes a single management entity.

scalable.: Pertaining to the capability of a system to adapt readily to a greater or lesser intensity of use, volume, or demand. For example, a scalable system can efficiently adapt to work with larger or smaller networks performing tasks of varying complexity. Generally, the term "scalable" is understood to mean that an entity can "grow." For example, an application can handle more users, or a mainframe can be made larger by adding more processors.

SCSI.: Small Computer System Interface. The protocol that is used by most non-mainframe servers to communicate with external storage devices. There are a number of SCSI standards that can be used.

server consolidation.: The process of taking a set of application serving capabilities and consolidating that into a smaller number. There are a few aspects of servers that lend themselves to consolidation. One can take a number of instances of like server function and consolidate them onto a single larger capacity server (for example, for mail or file or Web page serving), reducing the number of logical servers. Another type of server consolidation is to use a single instance of z/VM to rehost the set of servers onto one larger hardware server, that is, the zSeries processor, keeping the number of logical servers the same but reducing the number of hardware servers.

SLA.: Service-level agreement. An agreement or contract between a provider of services and a customer of those services, which sets expectations for the level of service with respect to availability, performance, and other measurable objectives.

SMP.: Symmetric multiprocessing. SMP is the term used to define a specific architecture for having multiple processors cooperate on a set of work. SMP means that the processors are symmetric in capabilities and share at least a common central storage.

SNMP.: Simple Network Management Protocol. A standardized protocol used to manage entities on a network, everything from printers and routers to servers. SNMP is an application layer protocol. Information on devices managed is defined and stored in the application's Management Information Base (MIB).

socket.: An endpoint provided by the transport service of a network for communication between processes or application programs.

SSL.: Secure Sockets Layer. A security protocol that provides communication privacy.SSL enables client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, and message forgery. SSL was developed by Netscape Communications Corp. and RSA Data Security, Inc.

subchannel.: (1) A division of a channel data path. (2) The channel facility required for sustaining a single I/O operation. (3) The facility that provides all of the information necessary to start, control, and complete an I/O operation.

throughput.: A performance measurement. Work done per unit time.

tool.: Any object or (software) becomes a "tool" when, in the perspective of its user, the object enables the task at hand to be performed more easily.

TSO.: Time Sharing Option. An integrated function of the z/OS operating system that provides interactive time sharing from remote terminals.

vertical growth.: Growing by increasing the capacity of a resource. Contrast this with horizontal growth (growing by adding yet another instance of the resource).

VIPA.: Virtual IP Address. Is a feature on certain of IBM's mainframe network adapter cards. VIPA eliminates a host's dependency upon individual network adapters by virtualizing the relationship between network adapters and IP addresses. As a result, adapter failures do not affect active connections.

virtual memory.: A programming concept implemented by many operating systems in which there is a virtual model of memory used by programs, which expect a particular behavior (typically, byte-of-word addressability across some range of addresses.) The underlying operating system provides this construct to each of the programs, while mapping each program's virtual address to some real storage address on the actual hardware.

Virtual Machine (VM).: (1) A virtual data processing system that appears to be at the exclusive disposal of a particular user, but whose functions are supplied by sharing the resources of a real data processing system. (2) In z/VM, the virtual processors, virtual storage, virtual devices, and virtual channel subsystem allocated to a single user. Synonymous with virtual configuration. (3) In z/VM, the functional equivalent of a z/Architecture system. Each Virtual Machine is controlled by an operating system, such as CMS. CP controls the concurrent operation of several Virtual Machines on a processor complex.

VPN.: Virtual private network. A network comprised of one or more secure IP tunnels connecting two or more networks.

VSE/ESA.: Virtual Storage Extended /Enterprise Systems Architecture. The generalized term that indicates the combination of the DOS/VSE system Control Program and the VSE/Advanced Functions licensed program.

working set.: The set of virtual pages a program needs to begin/resume executing, and also the set of virtual pages a program will reference during its next "time slice" on the machine. It is important that operating systems correctly restore a program's working set before dispatching the program. If not, the program will "page fault" and not use its time slice. "Thrashing" might occur if there is no way for the operating system to use the history to predict which set of pages the program will next need. In this case, the program simply continues to "page fault" and makes little progress in completing its execution.

workload consolidation.: To move the functions of a given workload from many servers to fewer or even one server.

z/OS.: IBM's flagship operating system for the zSeries mainframe, which was designed and developed to meet the demanding quality of service requirements for enterprise e-business.

z/VM.: IBM's zSeries operating system that provides the Virtual Machine capability that allows hundreds or more guest operating systems to run on a single mainframe.