-
Ablative
- Ablative technology utilizes heat to remove a layer of some material. In
this context, it relates to writing information with a laser by using the
lasers heat to burn away a layer of the recording medium, thereby representing
a binary value.
-
Access pattern skew
- This refers to the tendency for reference to information to follow a
pattern whereby, information is referenced from a certain area for some time,
then another area, then a further one, rather than completely random jumps.
Thus, if information in a file was written sequentially across a number of
disks, utilization would tend towards one disk at a time. If however, the file
data is split into blocks, and each block written to a separate disk, the skew
is eliminated.
-
Access time
- This refers to the total length of time from the initiation of a request
for data, to the start of the receipt of that data from a device.
-
Actuators
- An actuator is the mechanical assembly that is responsible for moving the
disk head back and forth across the disk surface.
-
Allocation group
- An allocation group consists of a set of i-node pointers to data blocks,
and the data blocks themselves. This is a file system entity used to improve
access to files within a file system based on locality of reference.
-
Areal density
- This defines the density at which individual bits can resolved by the read
head. This equates to the maximum bit density supported by the media.
-
Archive
- Archiving involves moving data from the location that it is usually
accessed from (normally fast, expensive storage), to lower cost storage such as
tape. Information is normally archived if access to it will be very infrequent.
Contrast with retrieval.
-
Asynchronous
- This refers to an operation that can occur independently of other
operations. An asynchronous communication for example, can be sent and then
other work initiated without waiting for a response. Contrast with synchronous.
-
Autochanger
- An autochanger is a mechanical device designed to load an remove media from
a drive automatically. Tape and optical libraries have autochangers.
-
Backup
- Backup involves taking a copy of data, usually on some form of removable
media, so that in the event that information is lost, it can be easily
recovered. Contrast with restore.
-
Bad block relocation
- When a write of a block of data to a disk occurs, some software (and in
some cases the hardware), is capable of detecting that the write failed
(usually with a read following the write to test). In this case, transparently
to the process that requested the write, the hardware or software can mark the
block as bad so that it will not be used again, and redirect the write to a
fresh block.
-
Banding
- Traditionally, writing of bits to a disk surface occurs in a regular
fashion; thus the further in toward the center of the disk, the less
information can be stored. Banding refers to a process of dividing the disk
surface into a number of concentric regions. As the disk write head moves into
regions closer to the center of the disk, the bit write frequency increases
proportionally, thereby maintaining the bit density.
-
Block
- A block is a unit of data to be written or read. There are various block
sizes, depending upon the media and software. Disk device drivers currently use
a block size of 512 bytes to write to the disk.
-
Bus
- A bus is a data and control path between devices. It consists of power
lines, a number of data lines, and a number of control lines. There are various
standards including Micro Channel and PCI.
-
Cache
- A cache is a area of extremely fast (usually expensive) memory that is used
to maintain frequently accessed information, or store information temporarily.
Caches are used in various parts of a computer system. In disk subsystem
controllers for example, writes to disk will actually occur to the cache so
that a completion return code can be quickly returned to the writing process.
The actual write will occur from the cache when the subsystem has time to
satisfy it. The CPU also maintains several caches where instructions and data
can be pre-loaded while the current instruction is executing.
-
Caddy
- A caddy is a removable casing that a CD-ROM is placed in before being
loaded into the optical drive.
-
CCW
- Continuous composite write describes the magneto-optical implementation of
WORM. Erasure and rewriting are prevented by simply not allowing the functions
to take place. Contrast with WORM.
-
CD-ROM
- A CD-ROM is an optical disk that has information stored on it before it is
distributed. The information is permanently stored and cannot be erased or
rewritten.
-
Command tag queueing
- Command tag queuing refers to the SCSI-2 implementation of
piggy-backing commands together to a device on the SCSI bus. This
effectively allows commands to be overlapped, thereby improving performance.
-
Compression
- Compression techniques utilize hardware or software implemented algorithms
that are able to reduce the amount of storage needed by data. The reduction in
space is dependent upon the data used, as well as the compression algorithm.
Contrast with decompression.
-
Data transfer rate
- This is the rate at which data can be moved from the host system to a
device. It is normally measured in KB/sec or MB/sec.
-
Decompression
- This is the process of restoring compressed data to its original state, so
that it can be used again. Contrast with compression.
-
Device driver
- A device driver is a piece of software written to assist in the management
of a specific device. Other software will use the device driver as the
interface to the device for reading, writing and control functions.
-
Differential
- This refers to the communications technique of transmitting information as
the difference voltage between two signals. Normally, information is
transmitted as a single signal and can be corrupted by noise from external
sources. Differential transmission means that both signal lines will be equally
affected by any noise, the difference between them, the actual information,
remaining constant.
-
Directory
- A directory is a file system entity that is used to organize related
information within file systems. The space allocated to a file system for file
storage can be subdivided into directories so that files can be more sensibly
organized. Directories can have sub-directories, thereby forming an
organizational hierarchy. Files themselves can be thought of as being located
within a specific directory, and access to them is defined by a path that is
the directory hierarchy leading to them.
-
Diskette
- A diskette contains a circular piece of magnetic material that is read and
written in the same way as fixed magnetic disk. Diskettes are designed to be
removed and easily transported and to enable this function, have lower
tolerances and hence less storage capacity than fixed disks.
-
Disk pack
- A disk pack contains a traditional fixed disk but can be removed in its
entirety from the support structure (power, cooling, and connection bus). It is
far less portable than a diskette, but can contain as much information as a
normal fixed disk.
-
Dispatch
- This refers to the action of taking a process that is waiting to use the
processor from its wait queue and loading it into the processor for execution.
-
Dump
- Should an unrecoverable error occur within the computer system, a dump
usually occurs before the system halts completely, if possible. The dump
consists of the contents of main memory and processor registers immediately
prior to the error.
-
Elevator seeking
- In order to maximize the efficiency of disk head seeks, a simple sort of
the request queue can be implemented in software or hardware. This will allow
the disk head to satisfy requests using the smallest possible head movements
moving into the center of the disk and then out again, thereby minimizing seek
times.
-
Fast and wide
- The SCSI-2 standard defines a data bus up to 32 bits wide, and with a 10MHz
cycle time. This is currently utilized at 2 bytes giving a data rate of
20MB/sec. Fast refers to the clock speed, and wide to the data bus.
-
Fault tolerance
- Systems that have total redundancy are called fault tolerant. This means
that all operation critical systems have a hot standby that can take over in
the event of failure of the primary component.
-
File system
- A file system is a high level entity that manages the storage of data.
Through the file system, information can be organized within directories and
files created, read, written, and erased.
-
Floppy disk
- This is an earlier term for a diskette. It originates from the fact that
older diskette designs utilized soft covers that were pliable.
-
Fragment
- A fragment is a subdivision of a file system block. Blocks can be divided
into a number of fragments (up to eight, dependent upon file system creation
parameters). The purpose of fragments is to minimize the disk space wastage
that occurs as a result of partially allocated blocks.
-
Frame
- Real memory in a computer system is divided into sections for ease of
manipulation. These sections vary in size depending upon the system
implementation, and are known as page frames. Under AIX, a page frame is 4KB in
size.
-
HDA
- The mechanical component composed of the disk head, actuator, motor and
platters, is known as the head disk assembly.
-
Helical scan
- This technology was introduced from the consumer video marketplace. Tape is
partially wrapped around a spinning recording head that is mounted at an angle
to the tapes direction of travel. This results in data tracks being recorded at
an angle across the tapes surface, thereby using the tape recording area most
efficiently, and maximizing capacity.
-
Hierarchical
- This refers to an organizational method that involves levels that are
accessed by moving from one to another starting at the top of the hierarchy. An
example is a directory tree in a file system, where files are found by
navigating from the top of the tree through a series of subdirectories, until
the file is found.
-
Hierarchical storage management
- At a higher level, treating the various storage technologies available as
levels, with disk being the level for interactive access, through optical for
intermediate, to tape for backup/archive, defines a storage hierarchy through
which data can be migrated according to space and usage requirements. The
process of managing this mechanism is known as hierarchical storage management.
-
IDRC
- This is a compression technique implemented on some IBM tape drives that
allows data being written to the tape to be compressed as the writing is being
done, thereby increasing the capacity of the tape medium. When data is read
back, it is decompressed as the read occurs.
-
I-node
- An i-node is a file system entity that is used to locate a files data on
the actual disk. It contains pointers to the physical disk blocks containing
the data.
-
JFS log
- Every action that occurs within a file system is recorded into a log known
as the journaled file system log. These actions include events such as opening
files, writes to files, and closing files. If the system should fail during
operation, upon reboot, the file systems can be brought back to consistent
states by replaying the information contained in the log.
-
Journaled file system
- The Journaled File System is the main AIX file system implementation. It
defines a file system with a JFS log.
-
Library
- A tape or optical drive coupled with an autochanger and racks of media
comprises a library. Media can be unloaded and loaded from the racks
automatically, on request from the host system.
-
Licensed program products
- All software products purchased for use on the RS/6000 are known as
licensed program products.
-
Linear
- Linear with regard to tape technology implies recording information in a
straight horizontal direction along the length of the tape.
-
Locality of reference
- If when a process is executing, most of the time it runs is spent in
several small sections of the code, with occasional jumps to other parts, then
the process is said to display good locality of reference. This is beneficial,
as it means that the operating system can maintain the small number of most
utilized sections of the process in memory, and thereby achieve good
performance.
-
Logical partition
- A logical volume is composed of a number of logical partitions. Each
logical partition maps directly to from one to three physical partitions where
data is actually stored.
-
Logical volume
- A logical volume is an area of physical disk storage comprising a number of
logical partitions. Logical volume can be written to directly, or a file system
can be created within them.
-
Logical volume device driver
- The logical volume manager device driver is one of the components of the
logical volume manager. It implements the logical volume manager policies for
logical volumes.
-
Logical volume manager
- The logical volume manager is a collection of device drivers, disk data
areas, daemons, and management subroutines that collectively form a high level
interface to disk storage. It provides functions for the creation,
manipulation, access, and deletion of logical volumes.
-
Longitudinal
- In terms of tape technology, longitudinal recording defines a mechanism for
data recording that involves writing linear tracks on the tape surface.
-
Magneto-resistive
- This technology is used to implement rewritable optical disk storage.
Magnetic material in the optical media surface is heated with a laser and then
its polarity altered with an electromagnet. The material used is such that a
low powered laser shined on the material is polarized in different directions
depending upon the magnetic polarity, thereby representing binary states.
-
Mirroring
- This refers to the practice of maintaining two or more concurrent copies of
information. All copies are updated for each write of the information. In the
event of loss of a copy, the data can still be accessed from another copy.
-
Mirror write consistency
- When mirroring is implemented and data is being written, there is a danger
that if the system should fail during the writing of the data, all copies may
end up in an inconsistent state. Mirror write consistency utilizes cache
storage to maintain the data to be written until all copies have been updated,
thereby ensuring consistency between copies.
-
MTBF
- Mean time between failure is a measurement of the reliability of hardware
components. It is calculated as total operating time of a group of components
making up a device divided by the number of components in the device that fail
over that period.
-
Multitasking
- This is the capability of a computer system to divide its time between
multiple processes or tasks, such that the processes all appear to be executing
concurrently.
-
NFS
- The network file system is an implementation of a remote file system. Files
stored on a remote machine are made to appear as if they were being accessed
from the local machine.
-
NIC
- Numerically intensive computing generally refers to applications that
require much intensive calculation, such as modelling or statistical analysis.
-
Non-volatile
- This typically refers to memory storage that is capable of retaining
information during periods without power.
-
OEMI
- This is a standard third party interface used to attach peripheral devices.
-
OLTP
- This describes a category of application that performs repetitive
processing of records, such as bank account processing.
-
Optical disk
- A storage medium that is read using optical technology, usually a laser.
There are a number of different writing mechanisms including ablative,
magneto-optical, and phase change.
-
Page
- Generally, a page describes a block of information. In AIX a page is 4KB.
-
Page fault
- When a page of an application that was waiting on some event, and has been
paged out, is required, the CPU will try and access it, and in so doing
generate a page fault as the page will not be in memory. The fault causes the
missing page to be located in page space and copied back into memory, probably
causing some other currently inactive page to be swapped out.
-
Page frame
- Main memory is divided into a number of pages frames, in AIX, these are 4KB
in size. Pages of information are then loaded into page frames.
-
Page space
- Main memory is finite,and soon becomes filled with the pages of many
executing applications. If at this point, more applications wish to run, there
would be no room and they would have to wait. Page space defines a pool of
storage on disk where pages of applications that are waiting on some event can
be temporarily stored to make room for other application pages.
-
Paging
- Paging describes the process of temporarily copying pages from main memory
to page space in order to free up memory page frames for other applications to
use.
-
Parallel write
- When mirroring is being used, parallel write means that data to be written
will be simultaneously scheduled to all copies. This is the quickest way to
implement mirroring, though a failure during write will result in no valid
copies. Contrast with sequential write.
-
Physical partition
- A physical volume is divided up into a number of physical partitions whose
size is defined when the volume group containing the disk is created. These
partitions are then mapped to logical partitions when a logical volume is
created.
-
Physical volume
- Before a physical disk can be added to a volume group, it must be defined
as a physical volume. This process assigns the disk a unique number by which it
will be identified, and creates some on-disk data areas which are used to store
information regarding the disks usage.
-
Physical volume identifier
- The unique number assigned to a physical volume is known as the physical
volume identifier.
-
Pipeline
- Pipelining refers to the process of pre-fetching instructions into an
instruction cache in order to speed up process execution.
-
Platter
- Inside a disk drive is a spindle that connects a number of disks coated
with a magnetic material. Read/write heads on arms are moved back and forth
radially over the disks, which are spun to allow a series of concentric tracks
to be written on each disk surface. Each such disk is known as a platter.
-
PPM
- This refers to the process of determining a signal value by its presence or
absence. Thus if the signal is there, it represents logic one, if not, logic
zero. With optical disk, a low powered laser utilizes the same mechanism to
read data, a returned signal being logic one, no returned signal logic zero.
-
PWM
- This refers to the process of establishing a signal value using the change
in state from present to absent. Thus a transition from absent to present is
logic one, a transition from present to absent is logic zero. Using this
technique means that data can be more densely packed as discrete signals are no
longer required. Optical technology uses this technique where dots on the disk
can actually be overlapped as it is only the change in state from a sequence of
returned signals to no signal (or vice versa) that indicates a binary value.
-
Quorum
- The logical volume manager implements a process known as quorum checking.
This is used to ensure that before a volume group can be made available for
use, over 50% of the disks in the group have valid VGDAs, indicating that they
contain uncorrupted data. The quorum is the number of disks required to
constitute more than 50% of the total disks in the group.
-
RAID
- RAID arrays are designed to increase performance or availability through
the implementation of one of the following modes of operation. RAID 0 stripes
data across the disks for maximum performance. RAID 1 pairs off the disks and
mirrors data on each disk. RAID 3 stripes data across the disks and uses one
further disk to record parity information to allow data to be reconstructed in
the event of loss of one disk. RAID 5 splits the data into blocks and writes
blocks sequentially across the disks, intermixing parity blocks with data
blocks.
-
Read ahead
- When an application wishes to access a data file, just the first few pages
of data are actually read into memory. As the file is used, more pages are read
in according to two system parameters, minpgahead, and maxpgahead. If the
operating system detects that the file is being accessed sequentially, then it
will read in minpgahead more pages when further pages are required. If access
is still sequential, the next time pages are required, minpgahead + 2 will be
read in. This value is incremented by two as long as the access remains
sequential up to maxpgahead thereby enhancing performance for sequential reads.
-
Redundancy
- Providing a duplicate component within a subsystem that can be switched in
and used in the event of failure of the primary component means that the
component has redundancy.
-
Restore
- Restoring is the process of copying information back from its safe location
(usually some form of removable media) to replace the original copy that has
somehow been lost. Contrast with backup.
-
Retrieve
- Retrieval is the process of moving data back from archive storage to its
original location where it can be accessed. Contrast with archive.
-
Rotational Latency
- When a block of data is to be read/written from a disk, the actuator moves
the read/write head to the track where the block is located and then waits for
the platter to rotate the start of the block underneath so reading/writing can
begin. This delay before the start of the block arrives is called rotational
latency.
-
Scheduler
- The operating system maintains several priority queues of processes waiting
for their turn to execute on the processor. The scheduler is the operating
system component that decides which process is eligible to run next, and
selects it for dispatch.
-
SCSI
- The SCSI standards define a communications protocol and physical interfaces
to support the attachment of SCSI compatible devices to a host system, and
thence the devices controlled, and information read and written.
-
Seek time
- The seek time is the sum of the time taken for the disk head to be moved to
the required track plus the rotational latency.
-
Segment
- The total AIX address space of 4 petabytes is divided into segments of
256MB. There are several different types of segment including working,
persistent, client, and log segments.
-
Sequential write
- When mirroring is being used, sequential write means that data to be
written is scheduled to each mirror copy in turn, with the next not occurring
until the previous has completed. This method gives the highest chances of at
least one copy surviving in the event of failure during the write, but at the
cost of performance. Contrast with parallel write.
-
Serial
- When data is sent a single bit at a time (usually over two wires), the
communications is said to be serial.
-
Serpentine track interleaving
- This technology is an enhancement to longitudinal recording where the data
is written to tape in a series of blocks. The tape head is capable of
writing/reading several tracks simultaneously which it does during one pass
down the length of the tape. The head is then stepped laterally and the pass
restarted in the opposite direction. The stepping continues until the tape
width is full.
-
Single ended
- Single ended technology refers to non-differential communications where
information is transmitted serially using four wires, two to send and two to
receive.
-
SSA
- This defines a new communications protocol and physical interfaces for
connecting peripherals to the host system and communicating with them.
-
Stale
- When mirroring is being used, should one of the copies of the data fail,
then the copy is marked as stale, which reflects the fact that it can no longer
be considered accurate until it has been resynchronized with the other copies
after repairs have been effected.
-
Streaming
- When data is written continuously to a device in one long run, the data is
said to be streamed to the device, and the device itself capable of streaming.
-
Striping
- Splitting data to be written into equal sized blocks and writing blocks
simultaneously to separate disk drives is called striping the data and
maximizes performance to the disks. Reading the data back is also scheduled in
parallel, with a block being read concurrently from each disk then reassembled
at the host.
-
Stripe width
- The size of the block that data is split into for striping is known as the
stripe width.
-
Subsystem
- A subsystem is a collection of components that together perform some
function on behalf of the host system. An example is a RAID array subsystem.
-
Superblock
- A file system is split into a number of blocks whose size is 4KB in AIX.
The second and thirty first (a backup copy) are designated as the superblock
and contain administrative information regarding the file system such as
fragment size and overall file system size.
-
Swapping
- This is an alternative name for paging.
-
Swap space
- This is an alternative name for page space.
-
Synchronous
- This defines an operation that must occur with a fixed time relationship to
another operation. An example of this is synchronous communications where each
end maintains a clock, and data is sent at regular intervals, each clock tick
for example. Contrast with asynchronous.
-
TCP/IP
- This is a set of communications protocols that support the transmission of
information between computers.
-
Thin film
- This technology is used in the construction of read heads for tape and disk
devices. It allows a very high degree of sensitivity and a correspondingly high
bit density on the recording medium.
-
Throughput
- This defines the rate at which information can be transferred across an
interface and is a measure of performance. It is usually measured in KB/sec or
MB/sec.
-
Track
- This defines a single one bit wide stream of physical data written on a
storage medium. Tracks are concentric circles on disk and most optical media,
horizontal lines on longitudinal technology tape, and inclined lines on helical
scan technology tape.
-
Volatile
- Memory that does not maintain its contents during periods of no power is
known as volatile storage. Contrast with non-volatile.
-
Volume group
- This is a logical volume manager entity that contains a number of physical
volumes.
-
Volume group descriptor area
- Each physical volume has at least one VGDA stored on it. The VGDA contains
information regarding the organization and location of all logical volumes and
physical volumes within the volume group.
-
Volume group identifier
- Each volume group has a unique number identifying it known as the volume
group identifier.
-
Volume group status area
- Each physical volume has at least one VGSA stored on it. The VGSA contains
information regarding the status of all logical volumes and physical volumes
within the volume group.
-
Virtual memory manager
- This operating system component is responsible for managing memory
allocation and usage. The VMM manages the mappings between real memory, page
space and the file systems, and all addressing requests go through it.
-
WORM
- Optical media that utilizes a destructive writing process meaning that once
written, information cannot be erased. Contrast with CCW.