mirror of https://github.com/tLDP/LDP
569 lines
20 KiB
Plaintext
569 lines
20 KiB
Plaintext
<glossary id="glossary">
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<title>Glossary (DRAFT, but not for long hopefully)</title>
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<blockquote><para><quote>The Librarian of the Unseen University
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had unilaterally decided to aid comprehension
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by producing an Orang-utan/Human Dictionary.
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He'd been working on it for three months.
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It wasn't easy. He'd got as far as `Oook.'</quote>
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(Terry Pratchett, ``Men At Arms'')</para></blockquote>
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<para> This is a short list of word definitions for concepts
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relating to Linux and system administration. </para>
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<glossentry>
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<glossterm>CMOS RAM</glossterm>
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<glossdef><para>
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CMOS stands for "Complementary Metal Oxide Semiconductor".
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It is a complex technology, but put very simply it is a type
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of transistor which maintains its state even if computer is
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powered off. This is due to a small battery on the motherboard.
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As a result, it does not lose what was stored on it when the
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power is switched off.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>account</glossterm>
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<glossdef><para>
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A Unix system gives users <glossterm>accounts</glossterm>. It
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gives them a username and a password with which to log on to the
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account. A home directory in which to store files is usually
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provided, and permissions to access hardware and software. These
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things taken as a whole are an <glossterm>account</glossterm>.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>application program</glossterm>
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<glossdef><para>
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Software that does something useful. The results of using an
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application program is what the computer was bought for.
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See also system program, operating system.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>bad block</glossterm>
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<glossdef><para>
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A block (usually one sector on a disk) that cannot reliably hold
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data.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>bad sector</glossterm>
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<glossdef><para>
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Similar to <glossterm>bad block</glossterm> but more precise in
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the case where a block and a sector may be of differing sizes.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>boot sector</glossterm>
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<glossdef><para>
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Usually the first sector on any given partition. It contains
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a very short program (on the order of a few hundred bytes) which
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will load and start running the operating system proper.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>booting</glossterm>
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<glossdef><para>
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Everything that happens between the time the computer is
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switched on and it is ready to accept commands/input from
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the user is known as <glossterm>booting</glossterm>.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>bootstrap loader</glossterm>
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<glossdef><para>
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A very small program (usually residing in ROM) which reads
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a fixed location on a disk (eg. the <glossterm>MBR</glossterm>)
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and passes control over to it. The data residing on that fixed
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location is, in general, slightly bigger and more sophisticated,
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and it then takes responsibility for loading the actual operating
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system and passing control to it.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>cylinder</glossterm>
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<glossdef><para>
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The set of <glossterm>tracks</glossterm> on a multi-headed disk
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that may be accessed without head movement. In other words the
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tracks which are the same distance from the spindle about which
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the disk <glossterm>platters</glossterm> rotate. Placing data
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that is more likely to be accessed at the same time on the same
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cylinder can reduce the access time significantly as moving the
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read-write heads is slow compared to the speed with which the
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disks rotate.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>daemon</glossterm>
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<glossdef><para>
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A process lurking in the background, usually unnoticed, until
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something triggers it into action. For example, the
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<command>update</command>
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daemon wakes up every thirty seconds or so to flush the buffer
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cache, and the <command>sendmail</command> daemon awakes whenever
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someone sends mail.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>daylight savings time</glossterm>
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<glossdef><para>
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A time of the year during which clocks are set forward one hour.
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Widely used around the world in summer so that evenings have more
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daylight than they would otherwise.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>disk controller</glossterm>
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<glossdef><para>
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A hardware circuit which translates instructions about disk access
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from the operating system to the physical disk. This provides a
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layer of abstraction that means that an operating system does not
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need to know how to talk to the many different types of disks, but
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only needs to know about the (comparatively low) number of types of
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disk controller. Common disk controller types are IDE and SCSI.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>emergency boot floppy</glossterm>
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<glossdef><para>
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A floppy disk which can be used to boot the system even
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if the hard disk has suffered damage on its filesystem.
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Most linux distributions offer to make one of these during
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installation, this is highly recommended. If your Linux
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distribution does not offer this facility then read the
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Boot floppy HOWTO, available at the LDP (**Find URL to cite**).
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>fibre channel</glossterm>
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<glossdef><para>
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A high speed networking protocol primarily used in Storage
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Area Networks. Unlike it's name suggests, fibre channel can be
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ran over fiber optic, or copper cables.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>filesystem</glossterm>
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<glossdef><para>
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A term which is used for two purposes and which can have two
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subtly different meanings. It is either the collection of
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files and directories on a drive (whether hard drive, floppy,
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Cd-ROM, etc). Or it is the markers put onto the disk media
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which the OS uses to decide where to write files to (inodes,
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blocks, superblocks etc). The actual meaning can almost
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always be inferred from context.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>formatting</glossterm>
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<glossdef><para>
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Strictly, formatting is organizing and marking the surface of
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a disk into <glossterm>tracks</glossterm>, <glossterm>sectors
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</glossterm>, and <glossterm>cylinders</glossterm>. It is also
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sometimes (incorrectly) a term used to signify the action of
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writing a <glossterm>filesystem</glossterm> to a disk (especially
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in the MS Windows/MS DOS world).
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>fragmented</glossterm>
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<glossdef><para>
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When a file is not written to a disk in contiguous <glossterm>
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blocks</glossterm>. If there is not enough free space to write
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a full file to a disk in one continuous stream of <glossterm>
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blocks</glossterm> then the file gets split up between two or
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more parts of the disk surface. This is known as <glossterm>
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fragmenting</glossterm> and can make the time for loading a
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file longer as the disk has to seek for the rest of the file.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>full backup</glossterm>
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<glossdef><para>
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Taking a copy of the whole filesystem to a backup media
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(eg tape, floppy, or CD).
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>geometry</glossterm>
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<glossdef><para>
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How many cylinders, sectors per cylinder and heads a disk
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drive has.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>high level formatting</glossterm>
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<glossdef><para>
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An incorrect term for writing a filesystem to a disk. Often
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used in the MS Windows and MS DOS world.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>incremental backups</glossterm>
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<glossdef><para>
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A backup of what has changed in a filesystem since the last
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<glossterm>full backup</glossterm>. <glossterm>Incremental
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backups</glossterm> if used sensibly as part of a backup regime,
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can save a lot of time and effort in maintaining a backup of data.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>inode</glossterm>
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<glossdef><para>
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A data structure holding information about files in a Unix
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file system. There is an inode for each file and a file is
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uniquely identified by the file system on which it resides
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and its inode number on that system. Each inode contains
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the following information: the device where the inode resides,
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locking information, mode and type of file, the number of links
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to the file, the owner's user and group ids, the number of bytes
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in the file, access and modification times, the time the inode
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itself was last modified and the addresses of the file's
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blocks on disk. A Unix directory is an association between
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file leafnames and inode numbers. A file's inode number
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can be found using the "-i" switch to ls.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>iSCSI</glossterm>
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<glossdef><para>
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A network storage protocol that enables the sending of SCSI commands
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over a TCP/IP network. Primarily used in Storage Area Networks.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>kernel</glossterm>
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<glossdef><para>
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Part of an operating system that implements the interaction with
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hardware and the sharing of resources. See also system program.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>local time</glossterm>
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<glossdef><para>
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The official time in a local region (adjusted for location around
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the Earth); established by law or custom.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>logical partition</glossterm>
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<glossdef><para>
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A partition inside an <glossterm>extended partition</glossterm>,
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which is ``logical'' in that it does not exist in reality,
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but only inside the logical structure of the software.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>logical volume manager (LVM)</glossterm>
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<glossdef><para>
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A collection of programs that allow larger physical
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disks to be reassembled into "logical" disks that can be shrunk or
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expanded as data needs change.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>low level formatting</glossterm>
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<glossdef><para>
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Synonymous with <glossterm>formatting</glossterm> and used in
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the MS DOS world so differentiate from creating a filesystem
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which is also known as formatting sometimes.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>mail transfer agent</glossterm>
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<glossdef><para>
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(MTA) The program responsible for delivering e-mail messages.
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Upon receiving a message from a <glossterm>mail user agent
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</glossterm> or another MTA it stores it temporarily locally
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and analyzes the recipients and either delivers it (local
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addressee) or forwards it to another MTA. In either case
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it may edit and/or add to the message headers. A widely used
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MTA for Unix is sendmail.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>mail user agent</glossterm>
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<glossdef><para>
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(MUA) The program that allows the user to compose and read
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electronic mail messages. The MUA provides the interface
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between the user and the <glossterm>mail transfer agent
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</glossterm>. Outgoing mail is eventually handed over to an
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MTA for delivery while the incoming messages are picked up
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from where the MTA left it (although MUAs running on
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single-user machines may pick up mail using POP).
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Examples of MUAs are pine, elm and mutt.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>master boot record</glossterm>
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<glossdef><para>
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(MBR) The first logical sector on a disk, this is (usually)
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where the BIOS looks to load a small program that will boot
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the computer.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>network file system</glossterm>
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<glossdef><para>
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(NFS) A protocol developed by Sun Microsystems, and defined in
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RFC 1094 (FIND URL), which allows a computer to access files
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over a network as if they were on its local disks.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>operating system</glossterm>
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<glossdef><para>
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Software that shares a computer system's resources (processor,
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memory, disk space, network bandwidth, and so on) between
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users and the application programs they run. Controls access
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to the system to provide security. See also kernel, system program,
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application program.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>partition</glossterm>
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<glossdef><para>
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A logical section of a disk. Each partition normally has its
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own file system. Unix tends to treat partitions as though
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they were separate physical entities.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>password file</glossterm>
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<glossdef><para>
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A file that holds usernames and information about their accounts
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like their password. On Unix systems this file is usually
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<filename>/etc/passwd</filename>. On most modern Linux systems
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the <filename>/etc/passwd</filename> file does not actually hold
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password data. That tends to be held in a different file <filename>
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/etc/shadow</filename> for security reasons. See manual pages
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passwd(5) and shadow(5) for more information.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>physical extents</glossterm>
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<glossdef><para>
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A term used to describe a the chunks a physical volume is broken
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down into when using the Logical Volume Manager.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>physical volume</glossterm>
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<glossdef><para>
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A term used an actual disk partition, usually in reference to the
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logical volume manager.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>platters</glossterm>
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<glossdef><para>
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A physical disk inside a hard drive. Usually a hard drive is
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made up of multiple physical disks stacked up on top of each
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other. One individual disk is known as a <glossterm>platter
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</glossterm>.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>power on self test</glossterm>
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<glossdef><para>
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(POST) A series of diagnostic tests which are run when a computer
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is powered on. Typically this might include testing the memory,
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testing that the hardware configuration is the same as the last
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saved configuration, checking that any floppy drives, or hard
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drives which are known about by the BIOS are installed and working.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>print queue</glossterm>
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<glossdef><para>
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A file (or set of files) which the print <glossterm>daemon
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<glossterm> uses so that applications which wish to use the
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printer do not have to wait until the print job they have sent
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is finished before they can continue. It also allows multiple
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users to share a printer.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>read-write head</glossterm>
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<glossdef><para>
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A tiny electromagnetic coil and metal pole used to write and read
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magnetic patterns on a disk. These coils move laterally against
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the rotary motion on the <glossterm>platters</glossterm>.
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>root filesystem</glossterm>
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<glossdef><para>
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The parent of all the other filesystems mounted in a Unix filesystem
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tree. Mounted as / it might have other filesystems mounted on it
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(/usr for example). If the root filesystem cannot be mounted then the
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<glossterm>kernel</glossterm> will panic and the system will not be
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able to continue <glossterm>booting</glossterm>
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</para></glossdef>
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</glossentry>
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<glossentry>
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<glossterm>run level</glossterm>
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|
<glossdef><para>
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Linux has up to 10 runlevels (0-9) available (of which usually only
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the first 7 are defined). Each runlevel may start a different set
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of services, giving multiple different configurations in the same
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system. Runlevel 0 is defined as ``system halt'', runlevel 1 is
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defined as ``<glossterm>single user mode</glossterm>'', and runlevel
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6 is defined as ``reboot system''. The remaining runlevels can,
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theoretically, be defined by the system administrator in any way.
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|
However most distributions provide some other predefined runlevels.
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|
For example, runlevel 2 might be defined as ``multi-user console'',
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and runlevel 5 as ``multi-user X-Window system''. These definitions
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|
vary considerably from distribution to distribution, so please check
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the documentation for your own distribution.
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</para></glossdef>
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</glossentry>
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|
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<glossentry>
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<glossterm>sectors</glossterm>
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|
<glossdef><para>
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|
The minimum <glossterm>track</glossterm> length that can be
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allocated
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to store data. This is usually (but not always) 512 bytes.
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</para></glossdef>
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|
</glossentry>
|
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|
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<glossentry>
|
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<glossterm>shadow passwords</glossterm>
|
|
<glossdef><para>
|
|
Because the <glossterm>password file</glossterm> on Unix systems
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|
often
|
|
needs to be world readable it usually does not actually contain the
|
|
encrypted passwords for users' accounts. Instead a shadow file is
|
|
employed (which is not world readable) which holds the encrypted
|
|
passwords for users' accounts.
|
|
</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>single user mode</glossterm>
|
|
<glossdef><para>
|
|
Usually runlevel 1. A runlevel where logins are not allowed except
|
|
by the root account. Used either for system repairs (if the
|
|
filesystem is partially damaged it may still be possible to boot into
|
|
runlevel 1 and repair it), or for moving filesystems around between
|
|
partitions. These are just two examples. Any task that requires a
|
|
system where only one person can write to a disk at a time is a
|
|
candidate for requiring runlevel 1.
|
|
</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>spool</glossterm>
|
|
<glossdef><para>
|
|
To send a file (or other data) to a queue. Generally used in
|
|
conjunction with printers, but might also be used for other
|
|
things (mail for example). The term is reported to be an acronym
|
|
for ``Simultaneous Peripheral Operation On-Line'', but according
|
|
to the <ulink url="http://www.tuxedo.org/~esr/jargon">Jargon File
|
|
</ulink> it may have been a backronym (something made up later
|
|
for effect).
|
|
</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>system call</glossterm>
|
|
<glossdef><para>
|
|
The services provided by the kernel to application programs,
|
|
and the way in which they are invoked. See section 2 of the
|
|
manual pages.
|
|
</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>swap space</glossterm>
|
|
<glossdef><para>
|
|
Space on a disk in which the system can write portions of memory
|
|
to. Usually this is a dedicated partition, but it may also be
|
|
a swapfile.
|
|
</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>system program</glossterm>
|
|
<glossdef><para>
|
|
Programs that implement high level functionality of an operating
|
|
system, i.e., things that aren't directly dependent on the
|
|
hardware. May sometimes require special privileges to run
|
|
(e.g., for delivering electronic mail), but often just commonly
|
|
thought of as part of the system (e.g., a compiler). See also
|
|
application program, kernel, operating system.
|
|
</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>time drift</glossterm>
|
|
<glossdef><para>
|
|
This is a term for a computers inaccuracy at keeping track of time.
|
|
All computers have some rate of error when keeping time. With newer
|
|
computers this rate of error is extremely small.</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>track</glossterm>
|
|
<glossdef><para>
|
|
The part of a disk <glossterm>platter</glossterm> which passes
|
|
under one <glossterm>read-write head</glossterm> while the head
|
|
is stationary but the disk is spinning. Each track is divided
|
|
into <glossterm>sectors</glossterm>, and a vertical collection of
|
|
tracks is a <glossterm>cylinder</glossterm>
|
|
</para></glossdef>
|
|
</glossentry>
|
|
|
|
<glossentry>
|
|
<glossterm>volume group</glossterm>
|
|
<glossdef><para>
|
|
A collection of physical volumes broken down into physical
|
|
extents, and available for use in logical partitions.
|
|
</para></glossdef>
|
|
</glossentry
|
|
</glossary>
|