Glossary

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.