GLOSSARY OF TERMS
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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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