GLOSSARY OF TERMS

[ A | B | C | D | E | F | G | H | I | K | L | M | N | O | P | Q | R | S | T | V | W | Z ]

A


Access: To store on and retrieve data from a disk or other peripheral device. The term access is used as a verb in the computer world, and it means to retrieve a file or part of a file from disk so that the data (in the file) can be looked at, modified, and so on. Users access files constantly.

Access Control List (ACL): In OpenVMS, a list that defines the kinds of access to be granted or denied to users of objects such as files, devices and mailboxes. Each access control list consists of one or more entries known as "access control list entries."

Address: The location on disk or in memory where information is stored. Just as addresses are used to distinguish houses on a street, the computer uses addresses to distinguish the exact locations of all the pieces of information stored either in memory or on the disk.

Allocate: To reserve a resource such as memory or disk.

Allocated: Reserved for use. Before data can be stored on the disk, space for the data must be allocated from whatever remains available. One cluster is the minimum amount that can be allocated.

Application: An application is a computer program which controls the computer system to perform some useful work for the user.

Architecture: The basic design of something, the way in which it is built; e.g., disk architecture, software architecture.

Arm: Disk heads are mounted on arms that hold the heads close to the platter surface at precisely the right point to read or write data. There may be one arm for each head, but on multiple-platter disks a single arm may support two heads; one for the platter above the arm and one for the platter below.

Average Access Time: This is not exactly equal to half the sum of the rotational latency plus the seek time, as the rotation and the seek occur at the same time. The average access time, therefore, is somewhat less than half the total.

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B


Bandwidth: The measurement of the transmission capacity of a communication channel, usually a set of wires within a cable. The higher the bandwidth, the more information can be transmitted on these wires. The bandwidth is measured in bits per second. The more bits (of information) per second that can be transmitted, the better, as this speeds up the flow of communication between computers in a network, for example.

Batch: 1. Group, or collection, of items. 2. A method of scheduling and executing programs in which the programs run with no programmer interaction. (The Digital Dictionary)

Binary: Binary means "having two and only two possible states" such as on or off, true or false, and so on. Taken from the Latin "bini," meaning two by two and "ary," meaning of, or pertaining to. Therefore, binary describes a numbering system for counting, where only two digits (0 and 1) are used.

Bit: BInary digiT. The smallest part of information in a computer. Groups of bits make up storage units in the computer. For example, it takes eight bits to make up a byte, which is equivalent to one character, whether it be a number, letter, punctuation mark, or even a space. In written communication, bits are represented as zeros and ones.

Bit Map, Bitmap: A binary representation (literally, a map) in which each bit or set of bits corresponds to a particular condition (such as "empty," "not empty") or object (such as a type of font or an image). A bitmap is a table or series of bits which indicate whether something is, or is not in a certain condition. An example would be a table of 1s and 0s, pre-determined to mean "yes" and "no," representing the ratio of girls to boys sitting in a classroom, where "yes" meant "girls," and "no" meant "boys." By looking at the bitmap, you could tell where the girls were sitting in relation to the boys. In the context of VAX computers, a bitmap is located at the beginning of the disk, and is a set of bits, one for each cluster (of blocks) on a disk, indicating whether or not those blocks are available for use.

Block: In OpenVMS, a block is a collection of 512 bytes. OpenVMS disks are formatted into blocks of 512 bytes each. A block is a unit of data storage space on a disk that equals approximately one third of a page of text. A block is 512 contiguous bytes of information (or empty disk space), and is the smallest addressable unit of data that can be transferred in an I/O operation on a VAX computer. A block of space on a disk is equivalent to a page of space in memory.

Boot: To cause the computer to start executing instructions. The term comes from "bootstrap" since bootstraps help you get your boots on, booting the computer helps it get its first instructions.

Boot Image: The program that boots the system. The instructions that tell the computer how to start itself up. See boot and image, definition 2.

Bottleneck: A slow down due to too much activity being requested when inadequate facilities for the requests exist. In relation to a computer system, this refers to a situation that occurs when many processes are slowed down because there are too many I/Os waiting to be handled by the disk.

Bucket: A storage structure of 1 to 32 disk blocks that is used to store and transfer data in files. Unlike a block, a bucket can contain only entire records.

Bug: Programming error occurring in software. The term was coined in the 1940's when a moth was found inside an early computer prototype, shorting its circuits.

Byte: When eight bits are considered together, they are referred to as a byte. A single eight-bit byte is the amount of computer storage typically used to store a single letter of the alphabet or other symbol of human communication. The word "animal" could thus be stored in six bytes of computer storage.

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C


Cable: The electronics in the disk drive are connected to circuitry in the computer by means of cables, which are no more than wires with a certain type of connector on each end. Often, the individual wires are color-coded for clarity.

Cache: 1. Dramatic performance improvements can be gained by placing a significant amount of memory inside the disk controller. This local memory is called a cache and is used to store data recently retrieved from the disk by the computer. Then, if the computer should happen to request exactly the same data again, the controller can service the request from the local cache at memory speed (microseconds) instead of at disk speed (milliseconds). 2. A very fast memory that can be used in combination with slower, large capacity memories, such as disks.

Cache Hit Rate: The rate at which I/Os are satisfied from the cache, rather than from disk. Each time an I/O is satisfied from the cache, it is referred to as a hit.

CD-ROM: Compact Disc Read-Only Memory. A compact disc format used to hold text, graphics, and high-fidelity stereo sound. As suggested by its name, the end user cannot write data to a CD-ROM, but can only read what is already on the CD.

Chip: A very small component that contains a large amount of electronic circuitry. Chips are the building blocks of a computer and perform various functions, such as serving the computer's memory or controlling other chips.

Chunk: Due to the way OpenVMS disks are structured, disks in a stripeset are divided into "chunks" rather than clusters, chunks consisting (usually) of more blocks than clusters.

Cluster: 1. Disk blocks are grouped into clusters, which are groups of blocks read and written as a unit. In other words, a cluster is the minimum allocation quantity for a disk. The cluster size, in terms of number of blocks per cluster, can be varied only by reinitializing the disk. 2. See VAXcluster.

Common System Disk: A System Disk is the disk which holds the operating system and all the files necessary to start the computer. A Common system disk is the system disk in the VAXcluster from which all the other nodes in the cluster boot.

Compatibility Mode: Some VAXes have the ability to run PDP-11 programs as if the VAX were a PDP-11 computer running the RSX-11M operating system. When operating in this way, the VAX is said to be in compatibility mode.

Computer: The simple definition of computer is: An electronic machine for making calculations, storing and analyzing information fed into it, and controlling machinery automatically.

Contiguous: Adjacent; placed one after the other.

Controller: Sometimes referred to as an interface, or storage controller. The controller, which is attached to the computer, decodes instructions from the computer and issues instructions to the disk drive to do what the computer has instructed. The controller also receives data and status information from the disk drive, which it passes on to the computer in a form the computer can understand. A single controller may service more than one disk drive. See also HSC.

CPU: Central Processing Unit. The computing part of the computer. In general use, the term CPU implies memory as well, since data must be stored in memory in order to be processed.

Cylinder: The tracks at the same radius on each platter, taken together, are referred to as a cylinder. If you visualized these tracks without any other part of the disk, they would form the shape of a hollow cylinder.

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D


Data Structure: A design for the way data is laid out in a file for access by a program. Not unlike the concept of basic building blocks, there are several different standard data structure designs which apply to different uses. Word processing documents, spreadsheets and database files are all examples of data structures.

DAU: Disk Analysis Utility. The DAU can provide anything from a quick summary to a very detailed analysis of the condition of a disk. It can be run at any time on any Digital-supported disk and it is not necessary to allocate the disk, dismount it, or stop users from accessing it.

DCU: Disk Compression Utility. DCU was an off-line style defragmenter for RSX systems.

DECUS: The Digital Equipment Computer Users Society. A world-wide organization of users of Digital products, including thousands of System Managers. Provides technical information exchange among its members.

Delete: To remove an item of data from a file or to remove a file from the disk.

Desktop Workstation: A high-performance, single user microcomputer or minicomputer.

Detached Process: A process which runs without further intervention from the user. The user can set it up, and then the process will go ahead and run without the user having to give it any more instructions.

Digital: The preferred name (by Digital) for Digital Equipment Corporation. Also known as DEC.

Direct I/O: An I/O that goes to disk to read or write data without going through a cache first.

Directory: A directory is "a file that briefly catalogs a set of files stored on a disk . . . " (The Digital Dictionary). From the user's point of view, a directory is a catalog of the names of files that are grouped in a particular way.

Disk: A thin, round plate with a magnetic surface coating on which data can be stored by magnetic recording. (The Digital Dictionary)

Disk Cartridge: A removable disk unit which consists of a case containing a hard disk or a floppy disk.

Disk Compression: Another word for defragmentation.

Disk Device Designation: In OpenVMS, a disk is referred to by a unique code consisting of letters and numbers that indicate exactly which disk is meant. In the example DUA0, "D" means it is a disk device, "U" indicates the type of disk, "A" indicates that it is the first controller for disks of that type on this computer (the second would be "B", the third "C" and so on), and "1" indicates that it is the second disk drive associated with that controller (the first would be "0").

Disk Drive: The combination of one or more spindles, arms, heads, platters and electronics into a single physical device for storing and retrieving data is known as a disk drive. The term drive is often used to refer to the disk drive.

Disk File Structure: The standard form used for arranging files on a disk.

Disk I/O Bandwidth: The bandwidth is the transmission capacity of a computer channel or communications line of any kind. In this case, the disk I/O bandwidth refers to the amount of disk I/Os that can be transmitted; i.e., the number of I/Os that can be transferred to and from the disk.

Disk Structure: The way data is organized on a disk.

Drive: See disk drive.

Driver: The set of instructions used to manipulate a controller is known as a driver. The driver resides at the lowest levels of a computer's operating system, where it can interact directly with the hardware. The driver interprets instruction codes of the disk controller into standardized instructions recognizable and usable by the more generalized parts of the computer system, and vice versa. The driver enables an application program to issue a generic "get data" instruction, for example, to the disk without having to concern itself with the peculiarities of that particular disk (the number of tracks, platters, and so on). A single disk driver may service more than one disk controller.

Dump: (Noun) A printable file showing the contents of memory, disk or tape, often occurring in response to an error condition. Often preceded by the word "crash" (as in crash dump). Useful for the analysis of what was taking place when a computer system crashes, for example. The computer literally "dumps" the data onto paper.

Dump/Header Command: A command used to analyze the contents of a file header with the OpenVMS Dump Analysis Utility, which, as the name suggests, is a utility used for analyzing dumps.

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E


Electronics: Electronic circuitry is required to sense and record the magnetism on the surface of the platters and to move the heads. This circuitry is commonly referred to as the electronics of the disk. The electronics communicate data between the physical disk and the computer.

Electronic "Disk": Another name for "Solid State" or "Semiconductor" disks. Memory chips (in a separate box) set up to emulate a hard disk.

.EXE: A file extension commonly used to show that the file is an executable file.

Executable: A program file in a language which the computer can understand that is ready to run. Any commands contained in that file can be executed by the computer. A non-executable file is a file that holds data, such as a user data file.

Extent: When OpenVMS allocates space on a disk for a file, it is not always possible to allocate all the needed space contiguously on the disk. Sometimes it is necessary to allocate part of a file in one place and the remainder in another. Files have been known to be allocated in dozens and even hundreds of pieces scattered around the disk. Each piece of a file so allocated is called an extent.

Extent Cache: A portion of the system's memory that is set aside solely for the use of the OpenVMS file allocation mechanism. The extent cache stores the addresses of deallocated (i.e. free) block clusters, making it fast for OpenVMS to find free disk space by reusing these same clusters. This saves the overhead of scanning the storage bitmap of a disk to find free space.

Extent Header: A header that contains the additional map area pointers of a file. A file header contains (in addition to other data) map area pointers and it can hold only so many of these. As a file becomes more and more fragmented it has to have more and more map area pointers in order for all the fragments of the file to be located. Therefore, at a certain point an extra file header must be created to hold these, hence the term (extent header).

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F


FCP: File Control Primitive. The FCP is the set of instructions that are used to control files.

Field: An area reserved for data. The smallest unit normally manipulated by a database management system. For example, in a personnel file the person's age might be one field and their zip code another.

File: Information stored on a disk is ordinarily stored in a file. In fact, for any OpenVMS disk using the ODS-2 structure, no information can be retrieved from a disk unless it is contained in a file. A file is "a collection of related records treated as a unit and usually referenced by a . . . name." (The Digital Dictionary)

File Fragmentation: File fragmentation refers to computer disk files that are not whole but rather are broken into scattered parts, while free space fragmentation means that the empty space on a disk is broken into scattered parts rather than being collected all in one big empty space.

File Header: "A block in the index file that describes a file on a . . . disk. Every file residing on the disk has at least one header, which provides the location of the file's extents." (The Digital Dictionary). All the information necessary to identify, secure, and access a file is contained in the header. The header is NOT stored as part of that file; it is stored, along with all other headers, in a special file on each disk, called INDEXF.SYS.

Filespec: File Specification. Reference to the location of a file on a disk, which includes disk drive, directory name, filename, and file type (or extension).

Files-11: The name of the disk structure used by the OpenVMS operating system.

File System: A method of recording, cataloging, and accessing files on a disk.

Fixed Disk: See hard disk.

Floppy Disk: A removable storage medium. It consists of a single, round disk of flexible, tape-like material housed in a square envelope or cartridge.

Formatted: The term formatted is used to mean that the disk has certain marks that trigger reading and writing of data in particular spots, allowing storage and retrieval of data in groups of a particular size. In the case of a Files-11 disk, the size of the data groups is 512-byte blocks.

Fragmentation: The word fragmentation means "the state of being fragmented." The word fragment, means "a detached, isolated or incomplete part." It is derived from the Latin fragmentum, which in turn is derived from frangere, meaning "break." So fragmentation means that something is broken into parts that are detached, isolated or incomplete.

Free: Not allocated.

Free Space Fragmentation: The condition in which free space on a disk is broken up into small pieces or areas and scattered around the disk.

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G


Goes Down: Computer industry slang for "stops running."


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H


Hard Disk: A metal disk covered with a magnetic recording material. It comes in removable and fixed varieties that hold from five to several thousands of megabytes of information.
tion.

Head: A tiny magnetic device capable of reading or writing magnetic bits of information on the disk surface. The platter spins near the head(s), so that a single track of recorded information is continuously passing under the head, available for reading or writing. The head never touches the surface. Rather, it floats on a cushion of air so thin that a human hair or even a particle of cigarette smoke cannot pass between the head and the surface. As foreign particles that small would cause the disk to fail, such disks are sealed in air-tight containers.

Header: See file header.

Hit: When an I/O request is satisfied from the cache, it is referred to as a hit.

Home Block: The block contained in the INDEXF.SYS file on a disk or tape that contains information such as what type of disk or tape it is, and establishes the characteristics of that storage medium.

HSC: Hierarchical Storage Controller. A controller which controls, organizes, and prioritizes the activities of the storage devices connected to the system.

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I


Image: 1. Duplicate information or data copied from one medium to another. 2. A program file of which an image (or copy) is made and placed in memory to run.

Incremental Backup: Backing up only files that have been changed since the last backup, rather than backing up everything.

Index File: The Digital Dictionary defines index file as "The file on a . . . volume that contains the access information for all files on the volume and enables the operating system to identify and access the volume." The index file is a catalog of all the files on a particular disk. In fact, the header of a file resides within the index file. All the information needed to access a file is contained here.

Indexed Files: Indexed files have an index that contains pointers to organized data records elsewhere in the file. In such a file, variable length data records are stored in buckets of a certain number of blocks each.

INDEXF.SYS: This is the name of the file which contains the headers for all the files on that disk. It also contains certain information critical to the system's ability to access data on that disk volume, like the location of the INDEXF.SYS file itself. INDEXF stands for index file. The file type, .SYS, indicates that this is a system file, reserved for use by the OpenVMS operating system. In a volume set, there is an INDEXF.SYS file on each disk in the set.

Initialize: The process by which the computer prepares a disk for handling user information. This process erases any information that was on the disk.

I/O (Input/Output): Refers to the transfer of data between the CPU and a peripheral device, such as a disk drive, or a printer. An I/O is a read or write transaction. A user or application either needs to read data from the disk (output) or write data to the disk (input).

I/O Bound: A condition in which it takes an excessive amount of time to get data in and out of the computer, as compared to the time it takes to process the data. This condition adversely affects the performance of the computer by slowing it down.

Input/Output (I/O) Request Queue: A driver has associated with it a queue for holding I/O requests. This queue is merely a data structure enabling the computer to store an I/O request while it carries on with its work without it having to wait for the I/O processing to complete.

Install: To place software in a computer and get it up and running.

Installed Image: An executable program that is kept in an "always ready" state. Part of the program is kept in memory and the program image file is kept open in order that the program can be started faster than it could be otherwise.

Intelligent Disk Subsystems: A disk subsystem which handles some of the work involved in accessing and managing files which would otherwise be done in the CPU. An intelligent disk subsystem is created by moving some of these functions (such as determining file placement on the disk) from the software into the disk controller.

Interrupt Stack: When an external event must be serviced by the computer operating system, an "interrupt" occurs. During this time, all processes are temporarily halted, while the operating system works to service the event. Then all the processes resume their operations. During the interruption, process information is stored on the "interrupt stack."

I/O Bottleneck: Refers to a situation that occurs when many processes are slowed down because there are too many I/Os waiting to be handled by the disk.

I/O Routine: A set of instructions that perform tasks related to I/O.

I/O Subsystem: See I/O and subsystem.

I/O Throttling: Refers to the act of evaluating the amount of CPU idle time and reducing the number of I/Os to the disk.

I/O Transfer: The actual carrying out of an I/O request.

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K


Kernel Mode: Kernel mode is the deepest operating mode in a computer system. So called because it is the central part, or core, of the computer. Device drivers operate at this level.

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L


LAVC: Local Area VAXcluster. A VAXcluster is a group of OpenVMS computer systems which can share disk-related resources. The computers also share certain other resources which ease the task of managing users and their system usage.

LBN: Logical Block Number. When the blocks on a disk are considered from a programming point of view, they are viewed as logical blocks. The address of a logical block on a disk is its Logical Block Number (LBN). LBN 0 (zero) is the first LBN on a disk. Logical blocks correspond one-for-one to physical blocks, but the logical block number might not correspond directly to the same physical block numbers.

Local: Directly connected to, as in local disk or local node.

Local Memory: Memory used by a single CPU, or allocated to one single program function.

Logical: In computer terms, logical means "conceptual." For example, a disk may be given a logical name, such as USERDISK, so that users do not need to know the hardware (or physical) name.

Logical Block Number: See LBN.

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M


Machine: A slang term for computer.

Magnetic: When something is said to be magnetic, it means that it is capable of storing a small amount of magnetism.

Mainframe: A large computer.

Map: 1. (Noun) A picture (or list of addresses) of the location of data or objects. 2. (Verb) To map an object (such as a file, for example) is to locate it physically on the disk by indicating the individual blocks in which it is stored. When an LBN is mapped to a PBN, the LBN can be located and any data stored on it can be retrieved.

Megabyte: One million bytes.

Memory: The computer's working storage that is physically a collection of chips. All program and data processing takes place in memory. The program instructions are copied into memory from a disk or tape. Memory can be viewed as an electronic checker board with each square on the board holding one byte of data or instruction.

Memory Chip: A chip that holds programs and data either temporarily or permanently.

Microsecond: One one-millionth of a second.

Millisecond: One one-thousandth of a second.

Monitor: A high-resolution display screen for output from a computer, camera, VCR, etc.

MOVEFILE Primitive: Relocating a file on the disk for purposes of defragmenting is a multi-step process. Doing some of the steps without doing the rest can result in a file that is confused, damaged or even lost. The solution is to isolate the critical steps that must be all completely done or none done at all and treat these as a single step. Such a group of steps treated as a unit is called a primitive. Beginning with version 5.5 of OpenVMS, this operation is called the MOVEFILE primitive.

MSCP: Mass Storage Control Protocol. A software program which acts as a server to make all locally connected disks available to all nodes in the cluster.

Multi-Header File: As its name implies, this is a file with more than one header or, to be more precise, with a header containing so many retrieval pointers they won't fit into a single one-block header. OpenVMS, therefore, allocates a second (or third or fourth) block in the INDEXF.SYS file to accommodate storage of the extra retrieval pointers.

Multiprogramming: The capability of running two or more programs at the same time without interference.

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N


Node: A single computer in a network or cluster of computers.

"(An) n-squared order problem": A problem that squares in difficulty for each increment in size of the problem. For example, a problem of size 2 has a difficulty of 2 x 2 = 4, while a problem of size 3 has a difficulty of 3 x 3 = 9. As the numbers grow large, the difficulty of the problem snowballs. A problem of size 20, though only ten times as large as the 2 problem, has a difficulty of 20 x 20 = 400, or 100 times the difficulty of the 2 problem.

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O


ODS-2: On-Disk Structure Level 2. The second generation disk file structure supported by OpenVMS.

Off-line: Not connected to or not installed in the computer. If a terminal, printer, or other device is physically connected to the computer, but is not turned on or in ready mode, it is still considered off-line.

On-line: 1. A peripheral device (terminal, printer, etc.) that is ready to operate. 2. An on-line computer system refers to a system with terminals and users.

OpenVMS: An operating system used on VAX and Alpha AXP computer systems. The VMS stands for Virtual Memory System. Open was added to the original VMS name in 1993 to illustrate the fact that VMS now runs on more that one type of computer.

OpenVMS Home Block: A block in the index file on a disk that contains information about the disk as a whole.

Operating System: A collection of programs that controls the execution of computer programs and performs system functions. It is the software which organizes a CPU and peripheral devices into an active unit for the development and execution of programs.

Overhead: Amount of processing time used by system software, such as the operating system, or database manager.

Override: (Noun) One action which is set up to override (i.e. change, or prevent from occurring) another action.

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P


Page: 1. (Noun) In OpenVMS, one page is 512 bytes, the same size as a block. In other systems, a page may be more or less than 512 bytes. A page is to memory as a block is to disk. 2. (Verb) The process of putting a page of memory on disk. See pagefile.

Page file, Pagefile: An area of a disk that is set aside to hold information intended to reside in the computer's memory. The information can be brought into memory from this file, a few pages at a time, for use whenever needed. This mechanism requires a much smaller amount of physical memory than would be required if the entire program were to be loaded into memory at once.

PAGEFILE.SYS: The name of the pagefile. The extension .SYS indicates that this is a system file.

PBN: Physical Block Number. The actual arrangement of information on the surface of a disk platter is referred to as a physical block. The physical block number (PBN) is an address used for identifying a particular block on the surface of the disk.

PDP-8: Programmable Data Processor-8. A 12-bit computer preceding the PDP-11. The PDP-8 is widely regarded as the first minicomputer, which, at that time, was simply defined as a computer which cost less than $100,000. Considering inflation, the cost of such a computer would be closer to $1 million today.

PDP-11: Programmable Data Processor-11. A 16-bit computer on which the VAX was based. The PDP-11 introduced virtual memory to the Digital world, although early versions of the PDP-11 lacked this capability.

Peripheral device: Any hardware device connected to a computer, such as a terminal, printer, disk drive etc.

Physical Block: The actual arrangement of information on the surface of a disk platter is referred to as a physical block. The physical block number (PBN) is an address used for identifying a particular block on the surface of the disk.

Physical Disk: The actual, real, touchable disk.

Platter: A disk may consist of one or more platters, each of which may be recorded on both sides. The platter spins like a phonograph record on a turntable.

Primitive: Relocating a file on the disk for purpose of defragmenting is a multi-step process. Doing some of the steps without doing the rest can result in a file that is confused, damaged or even lost. The solution is to isolate the critical steps that must be all completely done or none done at all and treat these as a single step. Such a group of steps treated as a unit is called a primitive.

Process: 1. (Verb) To manipulate data in the computer, regardless of what is being done, is referred to as processing. It could refer to updating data in a file, or displaying the data on the terminal screen. 2. (Noun) One individual complete manipulation of data, such as updating data in a file, logging on, etc. is referred to as a process.

Program: A collection of instructions that tell the computer what to do. Synonymous with "software."

Program Image File: See image file.

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Q


QIO: Queue Input Output. The OpenVMS operating system contains a mechanism for queuing (inserting) an I/O request to the queue of a driver. This mechanism is called the $QIO system service. The dollar sign indicates that this abbreviation is Digital's. QIO stands for "Queue Input Output," where queue is used as a verb.

QIO Operation: A single act of queuing the I/O requests.

Queue: A sequence of items waiting for service, like people in line at the checkout counter in a store.

Quorum: A quorum disk is one which substitutes for a VAX, acting as a node in a VAXcluster.

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R


RAM: Random Access Memory. The computer's primary working memory in which program instructions and data are stored so that they are accessible directly to the central processing unit (CPU).

RA81 Disk: A type (or model) of disk made by Digital.

Random: In a random access or direct access file, every record is the same size. Because of this, records can be deleted and replaced with new ones easily. An example of a direct access file is a bookshelf full of books. You can go directly to any book desired and withdraw it from the shelf. Fragmentation of the contents of such a file causes virtually no performance problems, as the file is designed to be accessed in random order and any new record is guaranteed to fit precisely within any free space in the file.

Realtime: Immediate response. Processing time that corresponds to the real world. Realtime video transmission produces a live broadcast.

Reboot: To reload the operating system and restart the computer.

Record: A collection of related data items treated as a unit. A record contains one or more fields. (The Digital Dictionary)

Removable Disk: A type of disk that is inserted into a disk drive for reading and writing and removed when not in use. Floppy disks and disk cartridges are examples of removable disks.

Reserved Field: A field which is reserved by Digital for possible future development. A segment of a data record that is set aside for special functions and cannot be used for anything else.

Retrieval Pointers: Sometimes referred to as Map Area Retrieval Pointers, or Map Pointers. Within the file header, the information critical to a discussion of file fragmentation is the section headed Retrieval Pointers. These pointers indicate where the file's data is located on the disk. Each pointer consists of the LBN of the first data block and a count of how many successive contiguous blocks contain data for that file.

Revectored: Revectored means that the LBN assigned to that physical block is reassigned to some other physical block. This revectoring can also be done on the fly while your disk is in use. The new block after revectoring might be on the same track and physically close to the original, but then again it might not.

RISC: Reduced Instruction Set Computer. By using simple sets of instructions, processing speed can be increased considerably. Digital's Alpha AXP computer uses RISC technology, as do other computers.

RK05: The RK05 disk, an older model disk, which held 2½ megabytes (5,000 blocks).

RMS: Record Management Services. The VAX file management system.

Rotational Latency: As a disk platter spins around the spindle, the blocks in a single track of recorded data are brought near a disk head. The head can only read or write a block when that block is immediately under the head. Accordingly, the time to access a block of data on the disk varies. It is much quicker to access a block that is currently or about to be under the head than it is to access a block that has recently passed under the head and is moving away. The block that has just passed under the head has to wait nearly a full rotation of the disk for another access opportunity. This delay is known as rotational latency.

RPM: Revolutions Per Minute.

RQDXn Disk Controller: A type of controller.

RSX-11: Resource Sharing eXecutive-11. An operating system used with PDP-11 computers which allowed several users to share the computer simultaneously.

RT-11: Real Time-11. An operating system used with PDP-11 computers.

RZ23: A model of disk drive made by Digital. In size, it is about 2 inches by 4 inches by 6 inches, and it has a storage capacity of 104MB (megabytes).

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S


Saveset: A container file holding the data saved during a backup operation.

Sectors: The surface of a disk is sectioned into parts. This sectioning is not a physical marking on the surface, but rather it is just an idea that the disk is so divided. These sections are called sectors or blocks. The term sector is more common to personal computers and VAX and Alpha AXP hardware, while block is common OpenVMS terminology.

Seek: The movement of a disk head from one track to another is called a seek. The time it takes for a head to seek is one of the most critical factors in determining the speed of a disk.

Seek Ordering: By keeping track of the exact position of the heads at all times, the controller can determine which one of multiple requests from the computer can be serviced in the shortest time. Then, instead of servicing the computer's requests in the order received, the controller can service first the requests for data nearest the heads and then the requests for data farther away. This is called seek ordering, which simply means putting the seeks in a better order.

Seek Time: The time it takes for a head to seek is the most critical factor in determining the speed of a disk. This is known as the disk's seek time.

Semiconductor Disk: See electronic disk.

Sequential: In a sequential file, every record except the first falls immediately after the preceding record. There are no gaps. An example of a sequential file is a music cassette. You cannot get to any selection without searching through the tape. Accordingly, sequential files are not subject to internal fragmentation. The situation simply cannot exist.

Server: A specialized part of the computer system. It takes over one specific function of the computer to provide better service to the other parts of the system and to the people using it. For example, "Printserver" is a computer in a network that controls one or more printers. It stores the print image output from the users of the system and feeds it to the printer one job at a time. See HSC and MSCP.

Shadow Set: A group of two (or more) identical disks combined so as to be treated by OpenVMS as a single disk equal in capacity to only one of the disks in the group. Each time a block is written to the shadow set, the same block is written to all the disks in the set.

SIR: System Improvement Request. Each year, DECUS surveys its members to determine the things that most need improvement in the OpenVMS operating system. This survey is called the System Improvement Request (SIR) ballot.

Spindle: A disk platter is attached to a spindle around which it rotates like a wheel on the axle of a car. The spindle is at the exact center of the platter. The arm moves the head from the outer edge of the platter toward the spindle at the center and back out again.

Split Transfer, or Split I/Os: This refers to how many times the VAX is having to do two or more I/O transfers when one would serve. A split transfer is the result of fragmentation.

SPM: Software Performance Monitor. A performance analysis tool from Digital Equipment Corporation. SPM provides limited fragmentation analysis capability.

SQUEEZE: Under the RT-11 operating system, the solution to the problem of not enough contiguous free space to create a file was the SQUEEZE command. SQUEEZE compacted the disk, rearranging the files so they were all together near the beginning (LBN 0) of the disk, leaving all the free space in one large, contiguous area at the end.

Stack: A reserved amount of memory used for arithmetic calculations for keeping track of internal operations. Called a stack because it handles things in a "first in, last out" sequence, like a stack of papers to be read, or plates to be washed.

Standalone: A single computer that is not part of a VAXcluster.

Stand-Alone Backup: A version of the OpenVMS BACKUP utility that runs by itself without OpenVMS running on the computer. Used to make backup copies of the system disk.

Storage Bitmap: Literally a map made up of bits, each of which represent an object or condition. In the case of a storage bitmap, the map would represent which blocks or clusters are free, and which are allocated.

Stripeset: Two or more disks can be combined into a stripeset. A stripeset is similar to volume set in that the full capacity of all the disks in the set is available for data storage, and in that the whole set is treated by OpenVMS as a single large disk. The main difference is that, while each file extent must reside entirely on a single disk in a volume set, in a stripeset each extent of the file is deliberately spread across multiple disks. One chunk resides on one disk, the next chunk in sequence resides on the next disk, the next chunk on the next disk, and so on, starting over at the first disk when the last disk in the set is reached.

Subdirectory: A directory within a directory. Seven levels of subdirectories are allowed in OpenVMS.

Subsystem: In OpenVMS, a set of instructions that implement a particular facility in the system such as the file subsystem, batch subsystem, I/O subsystem and so on.

SWAPFILE.SYS: The file that contains the data pertaining to files which have been written to disk to make extra room in memory.

SYS$MANAGER:*.LOG File: SYS$MANAGER: is a directory name. *.LOG refers to all the log files in that directory.

System Disk: The disk containing the program images and main data structures for the OpenVMS operating system, from which the operating system is loaded into memory at boot time.

System Root Directory: The top-level directory that contains system files and subdirectories. All files used by the operating system are in this directory or subdirectories under it. System root directories are named SYS0, SYS1, SYS2, etc. Those directories are reserved for the OpenVMS operating system.

System Software: Programs used to control the computer and run application programs.

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T


Third-Party: Software products made by companies other than Digital, but whose products are compatible with Digital's.

32-bit Architecture: A computer system with the capability of processing 32 bits of information simultaneously in each cycle of the computer. Like a 32-lane highway, the path on which data flows through the computer carries 32 bits of data at once.

Throttling: Regulating the flow or speed (of I/Os, for example).

Throughput: Speed with which a computer processes data.

Track: A single track is one strip of disk space beginning at one point on the surface and continuing around in a circle ending at the same point. The tracks are concentric rings, not a spiral like the grooves on a phonograph record. Each surface has many tracks.

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V


VAX: The brand name of a computer system made by Digital Equipment Corporation of Maynard, Massachusetts. VAX is a high-performance, multiprogramming computer system based on a 32-bit architecture. VAX stands for Virtual Address eXtension (The Digital Dictionary). The name indicates the ability of the hardware to give the user a limited amount computer memory to work with while simulating much more. See also Virtual Memory.

VAXcluster: A group of OpenVMS computer systems which can share resources such as disk and tape storage.

VMS: Virtual Memory System. See OpenVMS.

Virtual Address: The address of a location in a conceptual memory space that may or may not correspond to a location in the computer's physical memory, but which is translated by the computer in such a way as to make it appear that it does so correspond.

Virtual Memory: In the OpenVMS operating system, to give the appearance of having more memory than it does, the operating system software places some of the user's programs and data in peripheral storage (usually on a disk). When they are needed, the system "swaps" one or more blocks of data from the disk with one or more pages of data from memory which are not currently in use. This gives the appearance of having much more memory than there is in actuality.

VMB.EXE: The main boot program for the OpenVMS operating system.

Volume: In OpenVMS, the word volume refers to a structured (initialized) disk. When considering a disk as a logical (conceptual) unit of storage, rather than a physical unit of storage, it is referred to as a volume.

Volume Retention Dates: Volume retention dates are used by OpenVMS to determine when a file on the volume "expires." When a file is created, its expiration date is set to the current date and time plus the specified maximum retention time. Each time the file is accessed, the current time is added to the minimum time. If the sum is greater than the expiration date, the expiration date is recomputed. This allows the System Manager to determine which files are not being used and so might be candidates for archiving.

Volume Set: OpenVMS has the capability of treating one or more physical disk drives as one disk. This capability is implemented in software and does not involve any additional electronics or cabling. When two or more disks are so combined, the combination is referred to as a volume set. A volume set is a group of two or more disks combined so as to be treated by OpenVMS as a single disk equal in capacity to the total capacities of all the disks in the set.

VPA: VAX Performance Advisor. A performance analysis software tool from Digital Equipment Corporation. VPA provides limited fragmentation analysis capability.

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W


Window: The term window, as it relates to this book, means the set of retrieval pointers the system keeps in memory to access the file. If the file is contiguous, only one pointer is needed to access the file. A window typically holds seven pointers.

Window Turn: The situation in which OpenVMS had to load new retrieval pointers from a file's header to gain access to the desired portion of a file. If the file is contiguous, only one pointer is needed to access the file. A window typically holds seven pointers, so a file can be fragmented in up to seven pieces and still can be accessed with no further action on the part of the operating system. When there are eight or more pieces, however, one or more pointers have to be flushed out of the window and new pointers loaded to locate the later parts of the file. This is known as a window turn. If a file is fragmented into many pieces, window turns can become a major performance bottleneck.

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Z


Zero Length File: A file with no blocks allocated to it.

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