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Resources | Glossary


Access Control : General term for a group of security techniques such as using passwords or smart cards to limit access to a computer or network only to authorized users. 

Access time : The interval between the time a request for data is made by the system and the time the data is available from the drive.

Advanced Metal Evaporated (AME) : A type of tape media created by depositing a thin layer of magnetic material onto a base by means of evaporation. AME`s pure magnetic layer allows for high data density.

Advanced Servo Positioning (ASP) : This closed-loop positioning system allows increased data density by continually adjusting the position of the tape drive`s read/write head to achieve the strongest signal strength.

Allocation : The process of assigning particular areas of the media to particular data or instructions. 

AIT : Advanced Intelligent Tape; a magnetic tape format developed by Sony that uses 8-mm cassettes with built-in memory chips to speed the process of data retrieval. 

Archiving : Generally refers to long-term storage of important information on removable media. Tape media such as DLT is an excellent choice for archiving due to its 30-year shelf-life. 

Array (of disks) : A group of storage devices controlled in such a way as to provide higher data transfer rates, higher availability or both. The disk array is treated as a single volume by the operating system. 

ATL : Automated Tape Library. A tape cartridge storage device that includes a mechanism for unattended mounting and demounting of cartridges. ATLs can improve productivity, reduce costly tape-mount and pick-volume errors and provide a real time tape-volume tracking system. 

Automation (tape) : Refers to tape stackers, autoloaders, libraries used in data backup, archiving, hierarchical storage management, and near on-line storage operation. A tape automation system generally hasslots for multiple tape cartridges, a robotic cartridge-handling system, an electronic control system, software, and one or more tape drives. 

Autoloader : A basic (entry-level) tape automation system that uses a robotic mechanism to automatically load and unload tape cartridges into a single tape drive. Used to provide unattended data backup and file restoration.

Availability : The probability that a system is available at a given instant. In the context of a server, this function expresses the expected percentage of a time system is available to respond to client requests. 

Average Seek Time : The average time it takes for the read/write head to move to a specific location. To compute the average seek time, divide the time it takes to complete a large number of random seeks by the number of seeks performed. 


Backup/Restore : The act of copying files and databases to protect them in the event of a system failure or similar catastrophe and retrieving them at a later date. 

Backup Window : The period of time used to back up critical databases in off-line mode and other information that is changing. With an increasing number of organizatons running applications on a 7x24 basis, the backup window tends to be shrinking or becoming null. 

Backward Compatibility : The capability of all tape drives of a certain technology (e.g. DLT) to read tape media written on previous generations of the same techonolgy of tape drives. Preserves the end users` investment in DLT tape media and drives. 
Bad Data Block : A block that cannot reliably hold data because of a media flaw or damaged format markings.

Bar Code Reader : A scanning system used in many large tape libraries to inventory tape cartridges and identify cartridge content by recognizing assigned codes.

BBU (Battery Backup Unit) : A battery-operated power supply used as an auxiliary source of electricity in the event of power failure. The battery guarantees no lost writes and orderly transitions or shutdowns during power outages. 

BCV (Business Continuance Volumes) : This is EMC terminology refering to copies of active production volumes that are maintained by the disk system. These can be used to run simultaneous tasks using the same data, in parallel with one another. This gives users the ability to do concurrent operations, such as data warehouse loads and refreshes or point-in-time backups, without affecting production systems. 

Bezel : (Also known as the faceplate.) A plastic panel that extends the face of a disk drive so that it covers a drive bay opening. The bezel usually contains a drive-activity LED. 

Bit : Abbreviation for binary digit. A binary digit may have one of two values: 1 or 0. This contrasts with a decimal digit, which may have a value from 0 to 9. A bit is one of the logic 1 or logic 0 binary settings and corresponds to he smallest amount of data in a digital system. 

Block : A sector or group of sectors (in magnetic media data storage). By default, a sector of data consists of 512 bytes. 

Block-level I/O : Today’s standard interface between a file system and a storage device where storage blocks are identified by their location on disk. Block-level I/O breaks a file system request into many requests for pieces of data on a logical disk volume which are transferred between a server and storage device. Block-level I/O requires substantial processing by a host processor and also can require numerous disk accesses, each with a completion interrupt that must be processed on the host. Converse of file-level I/O where storage blocks are identified by their location within a file. 

BOT : Acronym for beginning of tape. Indicated on a DLT tape cartridge by a small hole in the leader portion of the tape. 

BPI : Acronym for bits per inch. A measure of how densely information is packed on a storage medium. 

Buffer : An area of RAM reserved for temporary storage of data that is waiting to be sent to a device. The data is usually on its way to or from the hard disk drive or some other peripheral device. 

BUS : A transmission channel in a computer or on a network that carries signals to and from devices attached to the channel. 

Business Continuance or Business Continuity : The technique of ensuring that a business or organization is able to weather a catastrophe due to natural disaster, human error or other cause, through the deployment of fault-tolerant and redundant hardware and software systems, alternative Data Centers, means and facilities to conduct critical operations etc.

Byte : The basic unit of computer memory, large enough to hold one character of alphanumeric data; comprises of eight bits. See also bit. 


Caching : A method of temporarily storing frequently accessed data in RAM or the memory buffer of a hard disk drive, to speed-up I/O processing. Computer systems and disk storage systems may also use write caching for write operatons, by using temporary memory buffers where data is held for a short time before being written permanently on the disk as a larger block. 

Cache : A memory buffer (e.g. RAM or FLASH) used to optimize data transfers between system elements with different performance characteristics, e.g., disk to main memory or main memory to CPU. 

Cache Buffering : A technique used in I/O operations with tape drives to reduce stops, starts, and repositioning delays due to a difference between the host data rate and that of the drive. 

Capacity : The amount of information that can be stored on a storage device (disk, tape etc). The data is stored in bytes, and capacity is measured accordingly (in megabytes, gigabytes, terabytes etc). 

Channel : A high bandwidth connection between a processor and other processors or devices. 

Checksum : A computed value that is dependent upon the contents of a data packet. This value is sent along with the packet when it is transmitted. The receiving system computes a new checksum based upon the received data and compares this value with the one sent with the packet. If the two values are the same, the receiver has a high degree of confidence that the data was received correctly.

Closed-Loop Servo Control : See Advanced Servo Positioning. 

Cluster : A collection of interconnected computer servers working together as a single processing resource or deployed in a failover configuration. Clusters provide scalable, high availability to both users and applications. 

Coercivity : The measure of magnetic strength required to change the magnetic orientation of a data bit stored on tape media. 

Compression : Encoding data to take up less storage space on magnetic media. Compression can be carried out in the host (software compression) or in the drive itself (hardware compression). Software compression speed is dependent on host processor power, whereas hardware compression gives optimum performance and is transparent to the user. In tape drive operations, data transfer speed and total tape capacity are affected by the data compression achieved. 

Compression Algorithm : A mathematical technique used to find patterns in data and thereby save space through encoding. Achievable compression is dependent on the type of data being compressed. All DLT tape drives use the LZ1 compression algorithm (See Lempel-Ziv). 

Connectivity : The ability of hardware devices or software to communicate with other hardware or software.

Controller : The chip or circuit that translates computer data and commands into a form suitable for use by a peripheral device such as a hard drive. 

Corrupted File : A data file that has been rendered unreadable due to a variety of causes, including tampering, accidental alterations, physical damage, and data loss. Corrupted files are one important reason why companies need to back up on a data regular basis. 

Cpio : A UNIX operating system command that stands for "copy file archives in and out." 

CPU : Central Processing Unit. The microprocessor chip or group of chips that perform the bulk of data processing in a computer. 

CRC : Cyclic Redundancy Check. An error detection code that is recorded within each sector and is used to see whether parts of a string of data are missing or erroneous. 

Cross-Platform : Systems that are operating-system independent and can operate across different system platforms. 


DASD : Direct Access Storage Device. Generic nomenclature for a storage peripheral that can respond directly to random requests for information. Usually refers to a disk drive. 

DAT : Digital AudioTape. A digital recording technology that uses a helical scan method of recording on tracks that are diagonal to the edge of the tape. Media are similar to that used in Audio recording (see also DDS).

Data Compression : A process generated by either hardware or software that reduces the amount of storage space required to hold a particular block of data.Typical compression rates are 2:1 or 3:1 and allow approximately double and triple the data storage respectively. 

Data Integrity : The accuracy of data after being transmitted or processed. 

Data Interchange : The capability of moving data from one tape drive to another without intermediate processing. 

Data Mart : A scaled-down version of a data warehouse that focuses on a particular subject area. Usually designed to support the unique business requirements of a specific department or business process. 

Data Mining : A means of extracting previously unknown, actionable information from the growing base of accessible data in data warehouses using sophisticated, automated algorithms to discover hidden patterns, correlations and relationships. 

Data Mirroring : Capability provided by multi-drive tape libraries or RAIT (Redundant Array of Independent Tape drives) systems whereby two copies of the same data are written simultaneously. Provides copy of data for off-site archiving and disaster recovery protection. 

Data Packet : In networking, a unit of information transmitted as a discrete entity from one node on the netwok to another. More specifically, in packet-switching networks, a packet is a transmission unit of a fixed maximum length that contains a header, a set of data, and error control information.

Data Warehouse : A systematic, comprehensive approach for collecting, cleaning, organizing and storing data, and making it available for retrieval as needed. 

DDS : Digital Data Storage is a format for storing and backing up computer data on tape that evolved from the Digital Audio Tape (DAT) technology. In 1989, Sony and Hewlett Packard defined the DDS format for data storage using DAT tape cartridges. Tapes conforming to the DDS format can be played by either DAT or DDS tape drives. However, DDS tape drives cannot play DAT tapes since they can`t pick up the audio on the DAT tape. The format was adopted as an industry standard for DAT drives by ANSI and others. Follow-on formats include DDS-2, DDS-3, DDS-4 and DDS-5. DDS uses a 4-mm tape. A DDS tape drive uses helical scanning for recording, the same process used by a video recorder (VCR).

Defragment : rearranging data on disk so that whole files are stored in contiguous sectors; used in order to improve file access.

Density : The amount of data stored in a particular section of media. The higher the density the more data stored in the same space. 

Device : A computer subsystem such as a printer, serial port, disk drive, or video adapter. Frequently, devices require their own controlling software (device drivers) to communicate with the computer system. 

Disaster Recovery : Preventative measures using redundant hardware, software, alternative data centers and other facilities to ensure that the organization will recover operations during a natural or man-made disaster. Restoring essential business operations as quickly as possible despite calamities and disasters is an essential part of Business Continuity planning.

Disk Controller : The hardware that controls the writing and reading data to and from and to a disk drive. It can be used with floppy disks or hard drives. It can be hard-wired or built into a plug-in interface board. 

Disk Mirroring : The duplication of disks and controllers so that two access paths exist in case a failure occurs on one of them. 

Disk Striping : Combining a set of same-size disk partitions from 2 to 32 separate disks into a single volume that virtually "stripes" these disks in a way that the operating system recognizes as a single drive. Disk striping enhances performance by enabling multiple I/O operations in the same volume to proceed simultaneously. 

Disk Striping with Parity : Preserving parity information across a disk stripe so that if one disk partition fails, its data can be re-created with information stored across the remaining portions of the disk stripe. 

DLT : Digital Linear Tape (DLT) is considered a de facto standard for magnetic tape technology used for computer data storage. It was invented by Digital Equipment Corporation in 1984, and was purchased by Quantum Corporation in 1994, who currently manufactures drives and licenses the technology and trademark.  DLT drive models include DLT2000, DLT2000XT, DLT4000,  DLT7000 and DLT8000 with increasing capacity and performance. Super DLT (SDLT) is a variant with higher capacity and performance.

DMA : Direct Memory Access. A means of handling data transfer between memory and a peripheral device that bypasses the central processing unit. 

Duty Cycle : A measure of the percentage of time a tape drive is actually used during a given period. Some tape drive manufacturers provide reliability figures based on 20 percent or less duty cycle. Other tape drive manufacturers (e.g. Quantum DLT) are based on 100 percent duty cycle. 


EDC : Acronym for Error Detection Code. DLT tape drives include a 16-bit EDC with every 4 KB of user data. The EDC helps the drive detect and recover any errors that may occur. 

EEPROM : Acronym for electronically-erasable programmable read only memory. An integrated circuit memory chip that can store programs and data in a non-volatile state. These devices, which are used to store firmware in DLT tape drives, can be erased and reprogrammed with new data. 

E-Infostructure : A shared foundation of technologies, tools, services, and intellectual capital that enables an uninterrupted flow of information 

EIDA : Enterprise Information Delivery Interface. An Enhanced IDE interface. 

EMC Proven E-Infostructure : An EMC program that recognizes leading corporations that operate in the 24-hour Internet workday and that adhere to the highest levels of information availability and customer satisfaction. 

Encoding : The protocol by which particular data patterns are changed prior to being written on the tape surface as a pattern of On and Off or 1 and 0 signals. 

Enterprise Storage : A combination of intelligent storage systems, software and services. Together, these products and services enable an enterprise to store, retrieve, manage, protect and share information from all major computing environments, including UNIX, Windows 2000 and mainframe platforms 

Erase : The removal of data from a piece of media. 

Error : A message that occurs when there is a loss of ability to interpret recorded data. Usually due to magnetic issues or defects in or on the media. 

Error Detection Code : See EDC. 

Error Correction Coding (ECC) : An encoding method that detects and corrects errors at the receiving end of data transmission. ECC is used by most modems. 

ESCON : Enterprise Systems Connectivity. A high-speed fiber-optic serial channel for IBM’s ES/9000 processors. Initially based in part of a fiber-optic link operating at a speed of 200 mbits per second but has been driven faster. 

ESN (Enterprise Storage Network) : It’s a specialized, open network that is designed to offer universal data access for every major computing platform, operating system, and application in the world across any combination of SCSI, Ultra SCSI, Fibre Channel, and ESCON® technologies. It integrates Symmetrix Enterprise Storage systems, EMC Connectrix, advanced, highly resilient network technology, and enterprise storage software with consulting and services into one complete package. An EMC ESN enables corporations to accelerate data access, boost network performance, automate storage management, and fully exploit the power of information regardless of its location. 

Fabric : A Fibre Channel topology with one or more switching devices. 

Failover : Data is immediately and nondisruptively routed to an alternate data path or device in the event of a failure of an adapter, cable, channel controller or other device. 

Fast Dump/Restore (FDR) : A family of mainframe-based backup/restore utilities that use Symmetrix with existing mainframe infrastructures to provide a comprehensive suite of fast, nondisruptive information protection solutions for both mainframe and open systems environments. 

Fault Tolerance : A computer or operating system’s ability to respond to a catastrophic event like a power outage or hardware failure so that no data is lost or corrupted. 

FCI : Acronym for flux changes per inch. See also BPI. Faceplate. See bezel. 

FibreAlliance : The FibreAlliance (www.fibrealliance.org) is an open association of industry-leading Fibre Channel vendors committed to accelerating the adoption rate of storage area networks (SANs). Members are working to develop a framework specification within which multiple vendors can develop integrated management environments for enterprise SAN customers. 

Fibre Channel Arbitrated Loop (FC-AL) : FC-AL places up to 126 devices on a loop to share bandwidth. Typically, this is done using a star layout that is logically a loop, employing a Fibre Channel hub. This allows IT managers to add or remove devices without having to bring the entire loop down. 

Fibre Channel (FC) : Fibre channel is nominally a one-gigabit-per-second data transfer interface technology, although the specification allows data transfer rates from 133 megabits per second up to 4.25 gigabits per second. Data can be transmitted and received at one-gigabit-per-second simultaneously. Common transport protocols, such as Internet Protocol (IP) and Small Computer System Interface (SCSI), run over Fibre Channel. Consequently, high-speed I/O and networking can stem from a single connectivity technology. 

FICON : Fibre Channel Connectivity. A standards-based IBM implementation of Fibre Channel. 

File-by-File Backup : A recording method that records data a single file at a time as opposed to mirroring a tape or block of data. 

File-Intelligent Storage Architecture : An integrated hierarchy of storage-related processes with the objective of achieving high performance and data accessibility through the use of file-intelligent I/O technology in which a file-intelligent controller transfers file-level requests to a peripheral storage device that has filesystem knowledge and can make decisions about where the request should be processed. 

File-Level I/O : The transfer of a file-level request from a host processor to a file-intelligent controller that contains the metadata necessary to identify the physical location of the data where request will be processed, resulting in a single request to the host. File-level I/O addresses data by the location within a file. This contrasts with block-level I/O that addresses data by its location on disk. 

Files : A distinct group of data blocks. 

Firmware : A category of memory chips that holds their content without electrical power and includes ROM, PROM, EPROM and EEPROM. Becomes “hard software” when holding program code. 

Flash Memory : Flash Erasable Programmable Read Only Memory. A non-volatile semiconductor storage device that can be reprogrammed electronically without removal from the circuit. 

Format : A magnetic track pattern that specifies the locations of the tracks and sectors. This information must exist on a tape before it can store any user data. Formatting erases any previously stored data. 

Formatted Capacity : The amount of room left to store data on a tape after writing the sector headers, boundary definitions, and timing information during a format operation. 

Form Factor1 : The industry standard that defines the physical, external dimensions of a particular device. 

Form Factor2 : The general geometric dimensions of a drive or width of a piece of tape media. 

FRAM : Ferroelectric Random Access Memory (Also FeRAM). A non-volatile computer memory chip that retains its content for up to 10 years without power. 

FRDS : Failure Resistant Disk Systems. New RAID Advisory Board classification for RAID storage systems. Replaces original numerical designations. Includes a premium “+” level. 

Full Height : Dimensions of a drive that meet standard height requirements (usually 3.25 inches for a tape or disk drive product). 


Gigabyte (GB) : A unit of measure consisting of one billion bytes (one thousand megabytes). 


Half Height : Standard drive size equivalent to half the vertical space of a 5.25-inch drive. 

Hard Disk : A mass storage device for computer data that consists of a hermetically sealed enclosure that holds stacked, rotating, magnetizable disks accessed by multiple read/write heads. 

Hard Error : A data error that persists when the tape is reread, usually caused by defects in the physical surface. 

Hardware RAID : Dual-storage processors that improve data availability and performance create data protection information and transfer it to the disk drives. They are located in an external storage subsystem, freeing the CPU from performing RAID parity, striping, and rebuild overhead calculations. This intelligent circuit board controls the disk drives. 

HBA (Host Bus Adapter) : An SCSI-2 adapter that plugs into a host and lets the host communicate with a device. The HBA usually performs the lower level of the SCSI protocol and normally operates in the initiator role. 

Head : The tiny electromagnetic coil and metal pole used to create and read back the magnetic patterns on the tape. Also known as the read/write head. 

Head Life : The length of time a tape drive head will function without replacement or repair, usually measured in hours of use. 

Helical Scan Recording : A method of recording data on tape in a diagonal motion. Increases track length, track density, transfer rates and total capacity. 

Hierarchical Database : A database that is organized in a tree structure, in which each record has only one owner. Thus, navigation to individual records takes place through predetermined access paths. 

Hierarchical Storage Management (HSM) : A method of storing mass amounts of data in tape libraries that allows for easy and rapid recall of the material. Due to the lowering cost of disk drives, HSM has yet to reach its full potential. 

Host : A computer server, typically networked, that runs applications used by or from other computers (e.g., web servers, file servers, and application servers). 

Hot Spares : In RAID systems, a spare drive in the disk array that is configured as a backup for rebuilding data in the event another drive fails. 

Hot Swapping : The process of removing and replacing a failed system component while the system remains online. 

HSM : Hierarchical Storage Management. The process of automatically storing data on the lowest-cost devices that can support the performance required by the applications. To users, data storage never fills; file-access, regardless of location in the storage hierarchy, is completely transparent. The software automatically manages multiple levels of storage hierarchy. 

Hub : A device joining communications lines at a central location, providing a common connection to all devices on the network. 


I2O - Intelligent Input/Output : Designed to facilitate the introduction of new I/O devices while offloading most I/O processing to intelligent I/O processors. Provides superior I/O performance by limiting the main CPU’s need to process basic input/output service requests and provides a consistent I/O layer so only one device driver version is needed across multiple operating systems. 

Image Backup : A backup option that takes a "snapshot" of an entire system by writing a volume image to tape sector-by-sector, rather than file-by-file. This method of backup is very fast and allows companies to backup critical information in a limited backup window. 

Information Management : The entire process of defining, evaluating, protecting, and distributing data within an organization. 

Infrastructure : The basic, fundamental architecture of a computer system. The infrastructure determines how the system functions and how flexible it is in meeting future demands. 

Intelligent : A device is intelligent when it is controlled by one or more processors integral to the device. 

Interface : A hardware or software protocol, contained in the electronics of the tape controller and tape drive, that manages the exchange of data between the drive and computer. The most common interfaces for small computer systems are AT (IDE) and SCSI. 

Internal Drive : A drive mounted inside one of a computer`s drive bays. 

Internet : A worldwide network of computer servers originally developed by the federal government as a communication system in the event of nuclear war or other wide-scale disaster. 

Intranet : A private version of the Internet that provides a cost-effective way to publish critical information and provide an interactive communication path for heterogeneous systems. 

Interoperability : The ability of hardware and software made by a variety of different manufacturers to work seamlessly together. 

ISA (Intelligent Storage Architecture) : EMC`s Intelligent Storage Architecture consolidates information management functions including backup/restore, disaster recovery, migration, and information sharing into a single enterprise storage system. This provides a single consistent platform from which to manage, access, and share information. 

ISV : Acronym for independent software vendor. 

JBOD : Just a Bunch of Disks. This refers to a disk drive array configuration in which there is neither redundancy nor intelligent controller. 

Jumper : A tiny connector box that slips over two pins that protrude from a circuit board. The jumper can be moved to change electrical connectors. When in place, the jumper connects the pins electrically. Some board manufacturers use dual in-line package (DIP) switches instead of jumpers. 

Kilobyte (KB) : A unit of measure consisting of 1,024 bytes. 


Lempel-Ziv Algorithm : A data compression technique used in all DLT tape drives. Named after Abraham Lempel and Jacob Ziv. 

Library System : A system that uses a robotic mechanism to automatically load and unload tape cartridges into one or more tape drives. Distinguishable from stackers and autoloaders in their ability to provide random access to tape cartridges. The DLTstor is a tape library system offered by Quantum Corporation. 

Linear Recording : Recording technology in which data is written in tracks that run the length of the tape media. Contrast with helical scan technology which records data diagonally across the tape. 

Link : A connection between two Fibre Channel ports. 

LTO : Linear Tape-Open. A half-inch open tape technology developed by IBM, Hewlett-Packard and Seagate. Consists of two different formats: Ultrium, a cartridge targeted at ultra-high capacity requirements; and Accelis, a dual-reel cartridge targeted for fast data access and automation applications. 

LUN (Logical Unit Number) : An encoded 3-bit identifier used on an SCSI bus to distinguish among up to eight devices (logical units) with the same SCSI ID. An LUN is an indivisible unit presented by a storage device to its host. LUNs are assigned to each disk drive in an array so the host can address and access the data on those devices. 

LUN Masking : An array security feature that lets a server access only its own and no other LUNs on a Fibre Channel. Each LUN can specify what host or combination of hosts has access to that LUN. 


MAPI : Messaging Applications Programming Interface. Programming Interface specification that enables an application to send and receive mail over a Microsoft Mail messaging system. 

Main frame : A computer primarily used by Global 2000 corporations for large-scale commercial applications. A mainframe is capable of supporting many users from many terminals. 

MB : See megabyte. 

Media : The material or device used to store information in a storage subsystem, such as a tape or disk drive. DLTtape media is a high-grade metal particle (MP) formulation that takes advantage of the latest advances in binder chemistry. By combining both solid and liquid lubricants in the tape binder system, tape and head wear are reduced while repelling airborne particles that could affect read/write head performance. In addition, by using a uniform particle shape, a dense binding system, a smooth coating surface, and a specially selected base film, Quantum DLTtape half-inch cartridge tapes take advantage of shorter wavelength recording schemes to ensure read compatibility with future generations of DLT drives. 

Megabyte (MB) : A unit of measurement equal to 1 million bytes. 

Megahertz (MHz) : A measurement of frequency in millions of cycles per second. 

Metal Particle (MP) tape : A magnetic recording media in which a flexible base is coated with a mixture of magnetic particles and a bonding agent. See also Media. 

Metadata : The information that is associated with a file but separate from data in the file; required to identify data in the file and its physical location on a disk. 

MHz : See megahertz. 

Microprocessor : The integrated circuit chip that performs the bulk of data processing and controls the operation of all of the parts of the system. 

Microsecond (µs) : One millionth of a second (.000001 sec.). 

Millisecond (ms) : One thousandth of a second (.001 sec.). 

Minicomputer : A somewhat out-of-date term used to describe a class of multi-user computer that was one notch below a mainframe system. Minicomputer popularity fell with the rise in popularity of the networked PC. Today`s server systems perform many of the functions that were once the domain of minicomputers. 

Mirroring : A data redundancy technique in which data is recorded identically and simultaneously on multiple separate disks. When the primary disk is off-line the alternate takes over, providing continuous access to data. Defined as RAID 1 configuration. 

Mission Critical : Applications that are vital to a company or organization`s well-being. 

Modularity : An approach to developing hardware or software that breaks projects into smaller units (or modules) that are deliberately designed as standalone units that can work with other sections of the program. The same module can perform the same task in another or several other programs or components. Modifying the way that module works will have no adverse affects on the other components of a program. 

MTBF : Acronym for mean time between failure. Reliability rating indicating the expected failure rate of a product in power on hours (POH). Since manufacturers differ in the ways they determine the MTBF, comparisons of products should always take into account the MTBF calculation method. 

MTTR : Acronym for mean time to repair. The average time it takes to repair a drive that has failed for some reason. This only takes into consideration the changing of the major subassemblies such as the circuit board or sealed housing. Component-level repair is not included in this number as this type of repair cannot be performed in the field. 

Multipathing : Multipathing allows for two or more data paths to be simultaneously used for read/write operations, enhancing performance by automatically and equally dispersing data access across all the available paths. 

Name Services Login : Worldwide-exclusive names that allow a device to log into the switch. 

NAS : Network-Attached Storage. Storage connected directly to the network, through a processor and its own operating system. Lacks the processor to run centralized share applications. 

Native Mode : Refers to the uncompressed storage capacity of a tape or disk subsystem. For instance, a DLT 7000 tape drive can store 35 GB in native mode and 70 GB with 2:1 compression. 

Near-On-Line Storage : An application that uses a tape drive or tape automation system in much the same way as a hard disk drive. Provides easy access to large amounts of critical information. 

NFR : Near Field Recording. A recording technology developed by TeraStor combining optical and current HDD recording technology. It promises recording densities up to 10 times that currently available.

Nonvolatile : Data in memory, cache and other electronic repositories are protected by a battery backup system to prevent their loss in the event of a power failure. 

NUMA : Nonuniform Memory Access. A form of SMPP, also known as shared memory cluster (SMC). A hybrid between symmetrical multiprocessing and clustering. Arranges multiple processors into small groups of processors, all of which communicate with each other. Designed to extend scalability beyond the SMP system bottlenecks. Memory is logically shared. 


OEM : Acronym for Original Equipment Manufacturer. 

Overhead : Command overhead refers to the processing time required by the controller, host adapter, or drive prior to the execution of a command. Lower command overhead yields higher drive performance. 

Overwrite : To write data on top of existing data thus erasing the original data. 

OLTP : Online Transaction Processing is a system that processes transactions the instant the computer receives them and updates master files immediately. OLTP is essential for good financial record keeping and inventory tracking. 

Parallel Channel Architecture : Allows DLT tape drives to read/write multiple channels simultaneously providing an fast data transfer rate (e.g. in the DLT 7000 tape drive). With this architecture, data blocks are not required to be located on any particular track or in consecutive order. This channel-independent block structure provides a powerful write-error handling system that allows bad blocks to be rewritten on the next available channel. 

Partial Response, Maximum Likelihood (PRML) : A technology that allows a disk or tape drive`s read channel to pack more data in the same amount of space on magnetic media. 

Parity : A data-error-checking procedure where the number of 1s must always be the same—either even or odd— for each group of bits submitted without error. Parity information is saved and compared with each subsequent calculations of whether the number is odd or even. 

Parity Bit : An extra bit used in checking for errors in transferred groups of data bits. In modem communications, it is used to check the accuracy of each transmitted character. In RAM, a parity bit is used to check the accuracy with which each byte is stored. 

Peak Transfer Rate : The maximum speed with which information moves inside a tape drive or between drive and host. Usually measured in megabytes per second. 

Peripheral : A device added to a system as a complement to the basic CPU, such as a disk drive, tape drive, or printer. 

PB (PetaByte) : One quadrillion bytes or one thousand terabytes. 

POH : Acronym for Power-on Hours. The unit of measurement for mean time between failure (MTBF), expressed as the number of hours that the system or drive is powered on. See MTBF. 

Port : On a computer, it is a physical connecting point to which a device is attached. 

PRML : See partial response, maximum likelihood. 

Protocol : A set of rules or standards intended to enable computers to communicate. 


QIC : Acronym for quarter inch cartridge. A tape storage subsystem that uses .25 inch wide media. 


RAID : Redundant Array of Inexpensive Disks.

RAID LevelCommon NameDescriptionDisk RequiredData Availability

Disk striping Data distributed across disks in array. No check dataNLower than single disk

MirroringAll data replicated on N separate disks (N usually 2) 2N, 3N, etc.Higher than single disk for read, similar for single disk for write
Parallel access disks with parity
Striped array with parity data stored on separate parity disksN+1Much higher than single disk; similar to RAID 5
5RAID 5 "RAID"Striped array with parity data stored on separate parity disks N+1Much higher than single disk; similar to RAID 3
6 RAID 6As RAID level 5, but with additional parity dataN+2Highest of all listed alternatives

RAIT : Redundant Array of Independent Tapes. A concept of striping data in parallel to multiple tape drives, with or without a redundant parity drive. Provide high capacity, high data transfer rate and data integrity. 

RAM : Acronym for random access memory. An integrated circuit memory chip that allows information to be stored and retrieved by a microprocessor or controller. The information can be stored or accessed in any order, and all storage locations are equally accessible. 

Random Access : The ability to directly locate any piece of data without having to read everything in memory or on disk. 

Read After Write : A mode of operation that has the computer read back each data block immediately after it is written on the tape, checking that the data read back is the same as recorded 

Read/Write Head : The mechanism by which data is recorded on to magnetic media in a tape or disk drive system. See Read After Write. 

Read-Only : Data can be retrieved (read) but not altered (written). 

Redundant : Backup arrays, drives, disks or power supplies that duplicate functions performed elsewhere. 

Reed-Solomon error correction : An error correction technique based on research done by Irving Reed and Gustave Solomon at MIT`s Lincoln Laboratory in the 1960`s. First used to check the accuracy of data received from the Voyager spacecraft. 

Restore : To replace data on the hard drive from another media device. 

Robust : Able to function or continue to function well in a variety of unanticipated situations. 

ROM : Acronym for read only memory. Integrated circuit chip containing programs and data that can be accessed and read but cannot be modified. 


SAN : Storage Area Network. A dedicated high-speed network that establishes a direct connection between storage elements and the back end of the server. 

Scalability : The capacity of hardware, software and networks to change size according to the number of users that they accommodate. Most often, scalability refers to the capacity to expand rather than shrink. 

SCSI : Small Computer System Interface. The standard set of protocols for host computers communicating with attached peripherals. SCSI allows connection to as many as six peripherals including printers, scanners, hard drives, zip drives, and CD-ROM drives. 

SCSI-2 : An enhanced ANSI standard for SCSI standard for SCSI buses. It offers increased data width, increased speed, or both. 

SCSI bus : A parallel bus that carries data and control signals from SCSI devices to an SCSI controller. 

Seek : The movement of a read/write head to a specific data track. 

Self Cleaning Head : Found in all DLT tape drives. Tiny ridges on either side of the DLT drive read/write head continuously wipe the tape clean as it passes over the head. This is why there is no periodic cleaning prescribed for DLT tape drives. 

Server : A powerful computer system with a large hard disk drive that serves the information access and communication needs of multiple users. Often servers are dedicated to a particular function such as Internet access, printing, file management, backup, and network communications. 

Servo Data : Magnetic markings written on the media that guide the read/write heads to the proper position. 

Shelf Life : The length of time that a tape can be stored without losing its magnetic strength. For DLTtape media, this period is 30 years or more. 

Shock Rating : A rating (expressed in Gs, which stands for multiples of gravity) of how much shock a tape drive can sustain without damage. Operating and non-operating shock levels are usually specified separately.

Self Cleaning Head : Found in all DLT tape drives. Tiny ridges on either side of the DLT drive read/write head continuously wipe the tape clean as it passes over the head. This is why there is no periodic cleaning prescribed for DLT tape drives. 

Server : A powerful computer system with a large hard disk drive that serves the information access and communication needs of multiple users. Often servers are dedicated to a particular function such as Internet access, printing, file management, backup, and network communications. 

Servo Data : Magnetic markings written on the media that guide the read/write heads to the proper position. 

Shelf Life : The length of time that a tape can be stored without losing its magnetic strength. For DLTtape media, this period is 30 years or more. 

Shock Rating : A rating (expressed in Gs, which stands for multiples of gravity) of how much shock a tape drive can sustain without damage. Operating and non-operating shock levels are usually specified separately.

SMPP : Shared Memory Parallel Processing. A symmetrical multiprocessing system in which memory is physically shared. 

Soft Error : A faulty data reading that does not recur if the same data is reread from the disk or corrected by ECC. Usually caused by power fluctuations or noise spikes. 

Stacker : A tape automation system that sequentially loads and unloads tape cartridges. Does not provide random access to cartridges or the data stored on them. 

Stepper : A type of motor that moves in discrete amounts with each electrical pulse. 

Software Raid: Uses the server processor to perform RAID calculations. Host CPU cycles that read and write data from and to disk are taken away from applications. Software RAID is less costly than dedicated hardware RAID storage processors, but its data protection is less efficient and reliable. 

Striping: The process of recording data across multiple disks rather than on one drive. Data is divided into segments each of which is written to successive drives. 

Supply Reel : The reel of tape contained within the DLTtape cartridge. DLTtape IV, for example, contains 1,800 feet of tape. Surface. The side of the tape that is coated with the magnetic material for recording data. 

Sustained Transfer Rate : The data transfer rate of a tape drive in native mode. For instance, the DLT 7000 has a sustained transfer rate of 5 MB in native mode, and up to 10 MB at 2:1 compression. 

Switch : A network device that selects a path or circuit for sending a data between destinations. 

System Manufacturers : Makers of computer systems and tape library systems. 

Symmetric Phase Recording (SPR) : A recording technique introduced with the DLT 7000 that writes data at alternating angles in a herringbone pattern thereby eliminating the need for guard bands between data tracks, and providing higher data density. 

Take-Up Reel : The reel inside every DLT tape drive onto which DLTtape media is wound. The in-the-drive take-up reel enables DLT tape systems to operate using a single-reel cartridge and thereby pack more tape and data into every cartridge. 

Tape Path : The path through which tape moves from the cartridge, past the read/write head, and onto the take-up reel. The patented DLT drive head guide assembly provides a gentle and solid path that ensures tracking accuracy and long tape life. 

Tar : A UNIX operating system command that stands for "create tape archives" and/or extract files. 

Terabyte (TB) : A thousand billion bytes or one thousand gigabytes. 

Throughput : In computers, it is a measurement of the amount of work that can be processed within a set time period. In networking, it is a measurement of the amount of data that can be successfully transferred with a set time period. 

Thin Film : A type of coating allowing very thin layers of magnetic material used on tape drive read/write heads. Media with thin film surfaces can store greater amounts of data. 

TPI : Acronym for tracks per inch. A DLT 7000 tape drive, for instance, writes data a density of 416 tpi, or 208 tracks across the width of the half-inch DLTtape media. 

Track : A linear or angled pattern of data written on a tape surface. DLT tape drives write information on multiple tracks simultaneously. 

Track-To-Track Seek Time : The time required for the read/write heads to move to an adjacent track. 

Transfer Rate : The rate at which the drive sends and receives data from the controller. Usually measured in megabytes per second. A DLT 7000 drive, for instance, has a native transfer rate of 5 MB/s. 

Travan : A tape technology that evolved out of the industry-standard QIC. Travan delivers more capacity than previous QIC formats. With capacities ranging from 400MB to 20GB, Travan formats are best for backing up individual machines or small servers. U

Unformatted Capacity : The total number of usable bytes on the media, including the space that will be required later to record location, boundary definitions, and timing information. See also formatted capacity. 

Volume : A virtual disk into which a file system, database management system or other application places data. A volume can be a single disk partition or multiple partitions on one or more physical drives. 

VTS : Virtual Tape Subsystem. Tape library hardware and software extensions that utilize DASD buffering to multiply the tape device count, throughput and storage density of tape library systems. 


Workload Balancing : It’s a technique that ensures no one data path can become overloaded while others have underutilized bandwidth causing an I/O bottleneck. When one or more paths become busier than others, I/O traffic shifts from the busy paths to the others, further enhancing throughput over the already efficient multipathing method. 

Write - Cache : A form of temporary storage in which data is stored (or cached) in memory before being written to a hard disk for permanent storage. Caching enhances overall system performance by decreasing the number of times the central processor reads and writes to a hard disk. 

Write - Mode : The state in which a program can write (record) information in a file. In the write-mode, the user is permitted to make changes in existing information. 

Write Once, Read Many (WORM) : An optical disk technology that allows the drive to store and read back data but prevents the drive from erasing information once it has been written. 


Zoning : Several devices are grouped by function or by location. All devices connected to a connectivity product may include configuration of one or more zones. Devices in the same zone can see each other; devices in different zones cannot.

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