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