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JBOD, What is JBOD, Disk Array, Fibre Channel


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What is JBOD? Just a bunch of disks. A Disk array without a controller.

Golosary of other RAID, JBOD, DISK ARRAY and storage related terms.


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Access Time: "Seek Time" + "Rotational Latency" = "Access Time" the time it takes to exactly locate data on a drive.

Actuator: A mechanical assembly that positions the read/write head assembly over the appropriate tracks.

Actuator Motor: The motor that moves the read/write head assembly over the appropriate tracks.

Array: Two or more hard disks that read and write the same data. In a RAID system, the operating system treats the array as if it were a single hard disk.

ATM: asynchronous transfer mode; a network architecture that divides messages into fixed-size units (cells) and establishes a switched connection between the originating and receiving stations; enables transmission of various types of data (video, audio, etc.) over the same line without one data type dominating the transmission.

Automatic Turbo FATs: Improve the speed of disk reads because NetWare 386 automatically indexes large files. A Turbo-FAT exists for cads randomly accessed file that is large enough to take up more than 64 FAT entries. By utilizing this method, NetWare does not need to scan every FAT entry searching for pointers to disk allocation blocks because the Turbo FAT tables store the pointers where those disk allocation blocks are assembled.


Backbone: The underlying network of communication conduit or line by which all main servers and devices are connected; backbone devices are typically servers, routers, hubs, and bridges; client computers are not connected directly to the backbone.

Backplane: A backplane is an electronic circuit board containing circuitry and sockets into which additional electronic devices on other circuit boards or cards can be plugged; in a computer, generally synonymous with or part of the motherboard.

Bandwith: The amount of data that can be transmitted via a given communications channel (e.g., between a hard drive and the host PC) in a given unit of time.

Bit(b): Bina digiT. A bi-state element which can only be either a "I" or a "0". Eight bits make up a byte of data. Lower-case "b".

Block: A portion of a volume usually 512 Bytes in size. Often referred to as a "Logical Block".

Block Interleaved: Evenly distributing the data at the bit level across 5 drives.See "RAID 0" and "RAID 5".

Bpi:Bits per inch or bit density.

Buffer, Track: Temporary storage area (RAM) residing on the disk drive that compensates for a difference in transfer rates and/or data processing rates; Can store the contents of an entire track. When the disk drive receives a request to read a specific sector, it reads the remainder of the track as well and places the data into the buffer. Thus, any request for sequential sectors from the PC can be serviced with the data already in the buffer, without requiring additional access to the disk surface(s). A track buffer is typically 256K or 512K, with some being as large as 1MB and is always flushed for the next non-sequential sector request: "Buffer" is often confused with "Cache".

Burst mode: A temporary, high-speed data transfer mode that can transfer data at significantly higher rates than would normally be achieved with non-burst technology; the maximum throughput a device is capable of transferring data.

Bus: A common pathway, or channel, between multiple devices.A bus allows for connecting multiple devices, whereas channels such as a PC's serial port can connect only to one device. CPU -- Located on PC motherboard; SCSI HBA plugs into it. I/O -- Data path between SCSI HBA and SCSI disk drive.

Bus mastering: A bus design that enables plug-in cards to run independently of the CPU and also to access the computer's memory and peripherals on their own.

Busmaster: Describes type of SCSI HBA that has its own on-board CPU chip; this allows the HBA to off-load all drive I/O commands from the server CPU and process them entirely independent of the server.

Byte (B): 8 bits = 1 Byte of data; a Byte of data is an ordered sequence of bits that represents a single alphanumeric or control (e.g., ESC ENTER, etc.) character.(Upper-case "B")


Cache: A large bank of random access memory used for temporary storage and information.

CAD: Computer-aided design; the use of a computer in industrial design applications such as architecture, engineering and manufacturing.

Capacity: The number of Megabytes (MB) of disk storage a drive can handle when matched with an operating system. See "Formatted Capacity".

CD-ROM:Cartridge Disk Read-Only Memory.Data is re-stamped onto a non-writable disk.Today there are "re-writable CD-ROMs.

Central Office: A secure, self-contained telecommunications equipment building that houses servers, storage systems, switching equipment, emergency power systems, and related devices that are used to run telephone systems.

Cluster:A contiguous group of sectors on a disk: the smallest unit of disk storage that DOS can manipulate.

Clustering: Creating a mini-network of computers that function in a fault resilient manner.If one computer fails, others in the cluster take up the additional load.

Clustered servers: The concept of combining multiple host computers together through a private communication line, such as Ethernet backbone, to form a ring of host computers. This ring of host computers acts as a single entity, capable of performing multiple complex instructions by distributing the workload across all members of the ring.

Clustered storage: The concept of combining multiple storage servers together to form a redundant ring of storage devices; clustered storage systems typically perform multiple read and write requests through parallel access lines to the requesting computer.

Commerce Service Provider (CSP): A company that provides e-commerce solutions for retailers.

Competitive Local Exchange Carrier (CLEC): A long distance carrier, cable company, or small start-up local exchange carrier that competes for business in a local telephone market. Many CLECs also offer Internet services.

Controller: A unit or circuitry that manages the information flow between storage disks and the computer.

Cost of ownership: The purchase price of equipment plus the cost of operating this equipment over its projected life span.

COTS: Commercial off-the-shelf, commercially available products that can be purchased and integrated with little or no customization, thus facilitating customer infrastructure expansion and reducing costs.

CPT: The amount of Command Process Time it takes for a drive to accept read and execute a command that has been sent to it be the host PC.

CPU: Central Processing Unit. The main circuit chip in a PC (or on a SCSI Busmaster HBA).

Crash: AKA "Head Crash" - Occurs when the read/write head collides with the disk latter, causing physical damage to the disk.

Cylinder: An imaginary construct formed by joining comparably numbered tracks on the drive's magnetic platters -- a stack of vertically-aligned tracks at one actuator position (precisely the same on each platter). A cylinder represents one track on each platter; the number of is equal to the number of tracks.


DAT: Digital AudioTape. A Digital magnetic tape format originally developed for audio recording and now used for computer backup tape. The latest DAT storage format is DDS (digital data storage).

Data availability: The amount of time a system needs to restore a failed network to full functionality.

Data currency: This refers to any discrepancies in a file before a crash and after the crash.

Data transfer rate: See "Transfer Rate".

Directory cashing: NetWare term. Decreases the time it takes to determine where a file is located on a disk. Both the FAT and directory entry are written into the file server's memory. The location of the file can then be read directly from memory, a much faster and more efficient process than reading from a hard drive.

Directory hashing: NetWare Term. Indexes file locations on a disk so that the time needed to locate a file is significantly reduced. This method increases directories in each volume and then indexes files by volume and sub directory thereby reducing the number of references made in search of a specific file.

Disc: A metal disk covered with a magnetic recording material. Each platter contains a number of circular recording tracks AKA "Platter".

Disconnect/Reconnect: Allows multiple SCSI disk drives chained together to perform simultaneous seeks, which results in multiple drives accessing, retrieving, and transferring data independent of one another and of the server. Major network improvement is gained by implementing this feature.

Disk array (or array): An arrangement of two or more hard disks, in RAID or daisy-chain configuration, organized to improve speed and provide protection of data against loss.

Distributed computing environment: A set of middleware standards that defines the method of communication between clients and servers in a cross-platform computing environment, enabling a client program to initiate a request that can be processed by a program written in a different computer language and housed on a different computer platform.

DLT: Digital Linear Tape. A serpentine technology first introduced by Digital Equipment Corporation and later developed by Quantum for tape backup/archive of networks and servers. DLT technology addresses midrange to high-end tape backup requirements.

DOS: Disk Operating System.

Drive group: Group of physical drives.

Duplexed: Two hard disks that have separate disk controllers and are mirror copies of each other. Data is written simultaneously to both.

Duplexing: Duplicating the data on one drive on a second drive identical drive using two independent data paths (i.e., using two paths separate HBAs cables, and terminators) for the complete data redundancy. See also "Mirroring".

DVD: Digital video disk.


EIA: Electronic Industries Association. A trade association, that establishes electrical and electronics-oriented standards.

EMI: Electromagnetic Interference. What occurs when electromagnetic fields from one device interfere with the operation of some other device.

Enterprise Storage Network (ESN): An integrated suite of products and services designed to maximize heterogeneous connectivity and management of enterprise storage devices and servers. A dedicated, high-speed network connected to enterprise's storage systems, enabling files and data to be transferred between storage devices and client mainframes and servers.

Erasable optical: Type of random-access storage device that uses laser technology to record data onto re-writable removable cartridges. May be either phase-change or magneto optical recording methodology.

Ethernet: A local area network standard for hardware, communication and cabling.


Failover: The transfer of operation from a failed component (e.g., controller, disk drive) to a similar, redundant component to ensure uninterrupted data flow and operability.

FAQ: Frequently Asked Questions. Usually a text file that gives the answers to common questions users have about a service, product or particular topic. Pronounced "Fak".

Fault tolerance: The ability of a network or device to handle failure. Actually, a system that has been set up with data redundancy capability such that if the data is lost from one disk drive, it can be recovered from another. See also "Duplexing" and "Mirroring".

FC-AL: Fibre Channel-Arbitrated Loop. A Fibre Channel implementation where users are attached to a network via a one-way ring (loop) cabling scheme.

FCLA: Fibre Channel Loop Association. An international non-profit organization whose members include manufacturers of servers, disk drives, RAID storage systems, switches, hubs, adapter cards, test equipment, cables and connectors, and software solutions.

Fibre Channel: A high speed storage/networking interface that offers a higher performance, greater capacity and cabling distance, increased system configuration flexibility and scalability, and simplified cabling.

Fibre Channel Arbitrated Loop (FCAL): This is the formal name for the Fibre Channel system used by SCSI. It is more commonly known, as Fibre Channel SCSI. The loop part of the name refers to the way the system is connected as one large ring. Because of the loop characteristics, this interface has more in common with local area networks than with parallel SCSI.

Fibre Channel SCSI: This refers to products with fibre channel physical and protocol layers using the SCSI command set. The Fibre Channel interface is completely different from parallel SCSI in that it is a serial interface, meaning command and data information is transmitted on one signal stream organized into packets. The fibre may be either a copper coaxial cable or a fibre optic cable. The signal on the first implementation of fibre channel uses a 1 GHz rate, thereby achieving 100 Mbytes/sec over the cable. Fibre channel also implements increased software control of configuration and pushes the total device on the bus to 126 Ids, as opposed to only 8 or 16 on a parallel bus.

File: A named ordered sequence of bytes that denotes cognizable information to the PC user.

Fixed (drive): Disks are mounted in a facto - sealed housing Hard Disks.

Flux (change): A state of magnetic resonance. One bit of data is represented on the disk surface as one flux reversal, which can be switched on or off by an electromagnetic head. A flux change (or reversal) induces a change in the direction of current of the read/write head coil, which is read as either a one or a zero. A flux change is the transition of the flux lines of the North and South poles of a magnet.

Footprint: The amount of floor space that a piece of equipment (e.g., a desktop/tower enclosure) occupies.

Form factor: The physical size and shape of a device. Often used to describe the size of disk arrays in a rackmount enclosure.

Format: Low Level Formatting is the process of dividing the tracks on a disk into sectors. Each track begins with an index mark, each sector has a sector identification field that is several bytes long, which contains the sector addresses and other overhead data required by the format. In addition, between the identification field and data fields of each sector are synchronization bytes, which allow the read/write head to synchronize with the rate of data transfer coming off the track. High-Level Formatting distinguishes operating systems (i.e., the difference between an IBM drive and an Apple drive).

Formatted capacity: The actual storage space on a disk drive that can be used to store files.

Fragmentation: Condition that occurs on a disk drive when files have been stored in noncontiguous clusters on a disk. Occurs, as files become too large to fit back into their original clusters. The effect is destraded performance caused by inefficient excessive seeks, cured via disk "optimization".

FRPI: Flux Reversals Per Inch.

FTP: File Transfer Protocol. The scheme that allows files to be transferred between computers on the Internet. A user can log onto a computer remotely through the Internet and request that a file be transmitted to that user's computer. FTP is usually done through an anonymous FTP.

Fusion: Take a simple example--a sequential file with a size of 32K, and NetWare preset for 4K block transfers. Without fusion, NetWare will handle the file status quo--"break" it up into eight 4K blocks, which now become requests in the I/O queue. Then, through such NetWare features, as split and elevator seeking, NetWare will handle the requests as it sees fit (i.e., in the order in which it deems is the most expedient). With the data fusion, the driver recognizes the eight requests residing in the I/O queue as being sequential that have been "broken up". It then takes that series of requests out of the queue, "fuses" them back together into a single request, and processes the entire file at the same time. Because a driver utilizing this feature does its own elevator seeking, it will appropriate each I/O request immediately after NetWare places it in the (NetWare) elevator queue. The data fusion feature can be dramatically illustrated through the use of NCOPY and identical hardware (only the drivers are switched). Running under NetWare 386, this feature can deliver performance increases by as much as 297%, depending upon the traffic of the file server. Maximum fused block size is 32MB.


Gigabyte: Approximately one billion bytes, 1,024 megabytes.


HBA: Host Bus Adapter. A hardware card that resides on the PC bus and provides an interface connection between a SCSI device (such as a hard drive) and the host PC.

Hard Error: A physical defect in the drive. May be caused by an indentation, foreign contamination (e.g., a dust or smoke particle), or an absence of magnetic material on a surface of a drive. A hard error cannot be recovered.

Head: The electromagnetic read/write device that detects/imposes flux transitions on the magnetic medium. For example, a disk with four heads has four magnetic surfaces, or platters.

Hot-Fix: Hot-Fix reassigns data to a new block during a disk write if NetWare determines that the block it is currently attempting to write to has been damaged. Hot-Fix does not recognize nor act on read errors.

Home page: The main page on a Web site that serves as the primary point of entry to related pages within the site and may have links to other sites as well.

Host-attached storage: A storage system that is connected directly to the network server. Also referred to as server-attached storage.

Hot set: In a RAID system, a set of replacement disk reading to go on-line instantly. Also called a Spareset.

Hot spare: A backup component (e.g., disk or controller) that is online and available should the primary component go down. When a disk crashes, the failure is detected and the array switches to the hot spare, automatically rebuilding data to that drive and using it as a replacement for the failed disk.

Hot swap: The process of replacing a component (e.g., disk drive, controller, fan, power source) with a new component while the rest of the system continues functioning without interruption.

HSM: Hierarchical Storage Management. A storage system in which new, frequently used data is stored on the fastest, most accessible (and generally more expensive) media (e.g., RAID) and older, less frequently used data is stored on slower (less expensive) media (e.g., tape).

HTML: Hypertext Markup Language. A page layout language that is interpreted by a Web browser. HTML describes title, text, hyperlink, button, graphic, etc. placement for a Web page.

Hub: A device that splits one network cable into a set of separate cables, each connecting to a different computer. Used in a local area network to create a small-scale network by connecting several computers together.

HVD: High Voltage Differential. A logic system used in some SCSI drives. It uses a paired plus and minus signal to reduce the effects of noise on the SCSI bus. Any noise injected into the signal would be present in both a plus and minus state, thereby being canceled.


IDE: Integrated Device Electronics. Another disk drive interface used in ISA and EISA PCs.

IEEE: Institute of Electrical and Electronics Engineers. The largest technical society in the world, consisting of engineers, scientists, and students. Has declared standards

Interface: Simply, the place in the electrical circuitry of the computer system at which the hard disk drive is connected to the controller. More precisely, the interface is a specification that defines the signals that must pass between the controller and hard disk. These signals include both the digital data and the analog control information that tells the hard disk where the data is or where it should go. The most common disk interfaces used in the PC environment today is SCSI.

Infrastructure: The physical equipment (computers, cases, racks, cabling, etc) that comprises a computer system.

Interleave: Occurs when a drive is set up so that its sectors are not consecutive. Interleaves other than 1:1 were used by older interfaces or drives that could not handle fast seek times.

I/O: Input/Output. Describes the entire connection path between the CPU Bus and the disk drive/. Also, describes the "read" and "write" information sent back and forth between the CPU and the disk drive.

IOPS: I/Os per second. A measure of performance for a host-attached storage device or RAID controller.

Internet: A worldwide system of linked computer networks.

Internet Service Provider (ISP): A company that provides Internet access services to consumers and businesses. ISPs lease connections from Internet backbone provides, while most ISPs are small companies that service a local area. There are also regional and national ISPs (such as America Online).

Interoperability: The ability of one computer system to control another even though the two systems are made by different manufacturers.

Intranet: A computer network, based on Internet technology, designed to meet the internal needs for sharing information within a single organization or company.

ISA: Industry Standard Architecture. PC bus type 16 bits.


JBOD: Just a bunch of disks. A Disk array without a controller.


Kernel: The core of an operating system such as Windows 98, Windows NT, Mac OS or Unix. Provides basic services for the other parts of the operating system, making it possible for it to run several programs at once (multitasking), read and write files and connect to networks and peripherals.


Landing zone: The Cylinder at which the heads are parked, which will least likely cause damage to recorded data if shock or vibration causes the heads to contact the disk surface.

LAN: Local Area Network. A network of computers within a limited area (e.g., a company or organization).

Latency: Describes the time it takes to service an individual I/O request.

Layered arrays: A large array that combines arrays instead of individual hard drives. This can improve performance and reliability but requires additional hard disks.

Legacy: A computer, system, or software that was created for a specific purpose, but is now outdated. Anything left over from a previous version of the hardware or software.

Load sharing: Also called Load Balancing. The use of multiple power supplies within a disk array so that power usage is distributed equally across all the power supplies. The failure of one supply will not cause the entire array to fail.

Logical Blocks: Blocks created for the purposes of duplexing two non-identical (capacity-wise) drives, the total of which, on the larger drive, are formatted to equal exactly the maximum total on the smaller drive.

Logical Drives: AKA "Logical Volumes". Disk drives, created in an extended partition that do not exist as separate physical drives, although the operating system treats them as if the logical units do exist.

Look-Ahead Buffer: The drive logic is capable of reading the data most likely to be requested beyond currently requested data. Access time is improved if the next data request is already present in the Look-Ahead Buffer. In a multi-tasking environment, however, a look-ahead buffer will only slow down overall system performance. Due to the random nature of file requests. See also "Cache" and "Buffer".

LUT: A device that translates between multiple devices and one SCSI ID.

LVD: Low Voltage Differential. A differential logic scheme using lower voltage levels than HVD.


Megabyte: Approximately one million bytes, 1,024 kilobytes.

MIA: Media Interface Adapters. MIA adapters are used to convert the copper I/O signal to optical. Fibre Channel enclosures support MIA on all port I/O connectors

Mirroring: Duplicating the data on one drive on the same data path (i.e., using the same MBA, cable, and terminator) for data redundancy. See also "Duplexing".

Mission critical: Any computer process that cannot fail during normal business hours. Some computer processes (e.g., telephone systems) must run all day long and require 100 percent uptime.

MSBF: Mean Swaps Between Failure. A statistical calculation used to predict the average usefulness of a robotic device (e.g., a tape library) with any interruption of service.

MTBF: Mean Time Between Failure, measured in hours. May be calculated from the combined MTBF times of the discreet components of a new drive, power supply, etc.. May be calculated from the actual field data of an older drive, power supply, etc.. May be calculated from in-house "burn-in" testing. In any case, this number is an artificial construct that does not tract a drive/power supply's true reliability, and as such, should be used carefully.

MTTR: Mean Time to Repair. The average amount of time required to resolve most hardware or software problems with a given device.

Multiplatform: The ability of a product or network to support a variety of computer platforms (i.e. IBM, Sun, Macintosh). Also referred to as a cross-platform.


NEBS: Network Equipment-Building System. Equipment standards set forth by Bell Communications Research (Bellcore) for electromagnetic compatibility, thermal robustness, fire resistance, earthquake and office vibration resistance, transportation and handling durability, acoustics and illumination, and airborne contaminant resistance.

Network-Attached Storage (NAS): A disk array storage system that is attached directly to a network rather than to the network server (i.e., host attached). It functions as a server in a client/server relationship, has a processor, an operating system or micro-kernel, and processes file I/O protocols such as SMB and NFS.

Network Service Provider (NSP): A company that provides the national or international packet switching networks that carry Internet traffic. Also called a backbone operator.

NIC: Network Interface Card.

Node (or network node): Any device that is directly connected to the network, usually through Ethernet cable. Nodes include file servers and shared peripherals.

NOS: Network Operating System.

NT (Microsoft Windows NT): An operating system developed by Microsoft for high-performance processors and networked systems.


OEM: Original Equipment Manufacturer. A Company that manufactures a given piece of hardware, unlike a value-added reseller, which changes and repackages the hardware.

Open systems network: A network comprised of equipment that conforms to industry standards of interoperability between different operating systems (e.g., Unix, Windows NT).

Operating systems (OS): The master control program (e.g., Windows) that manages a computer's internal functions and provides a means of control to the computer's operations and file system.

Optimization: Defragmentization. Rewriting all the files on the disk so that they are now stored in contiguous clusters. Effect is to lessen the number of seeks required to access a file, thereby increasing performance.

Overlapped I/O: See "Disconnect/Reconnect".


Parallel: During data transfer, the ability to transfer several things at the same time. For example, transferring 8 data bits at once rather than one at a time as with "Serial" data transfers.

Partition: "Carving" up a disk drive into multiple volumes. The section of a hard disk that contains an operating system.

Parity data: A block of information mathematically created from several blocks of user data to allow recovery of user data contained on a drive that has failed in an array. Used in RAID levels 3 and 5.

PCI: Personal Computer Interconnect. An industry-standard bus used in servers, workstations and PCs.

Performance matching: The disk drive, controller card, software (driver), and server all working together to exploit the maximum capability of each constituent to optimize efficient network operation. In other words, a network running leading-edge hardware does not necessarily mean that it is the fastest. If all the ingredients have not been designed to work smoothly together, performance will NOT follow.

Petabyte: 1,024 terabytes

Platform: A hardware standard, such as IBM, Sun or Macintosh.

Platter: A metal disk covered with magnetic recording material. Each latter contains a number of circular recording tracks.

Pre-Fix: The driver recognizes and interprets multiple read/write attempts as a potential for data failure. It verifies problem areas on the disk and reallocates the "bad" data blocks to "good" ones. Preventing data errors before they occur is important to the smooth operation of any network. A good driver will prevent both read and write soft or hard errors.

Proprietary: Privately developed and owned technology.

Protocol: A standard that specifies the format of data and rules to be followed in the data communication and network environments.

Physical Drive: Disk drive.


RAB: Raid Advisory Board. An organization of storage system manufacturers and integrators dedicated to advancing the use and awareness of RAID and associated storage technologies. Started in 1992, RAB states its main goals as education, standardization and certification.

Rack: The cabinet that houses a server/storage workstation (also referred to as a server rack). Used to mount equipment into a cabinet.

RAID: Redundant Array of Independent (or inexpensive) Disks. A collection of storage disks with a controller(s) to manage the storage of data on the disks. Two or more disk drives arranged in an array.

RAID 0: "Striping without Redundancy" - Really, "RAID O" is an anomaly - it has no redundancy, it is data striped across two or more disk drives with no fault tolerance. See also "Striping".

RAM: Random Access Memory. The art of a computer's memo to which the user has access.

RAM Disk: RAM set aside for use as a disk drive. All data is lost when computer is power-cycled, reset or warm-booted.

Random access: The ability to get to data stored anywhere on and in non-serial or random fashion.

Random: Not in an order. Refers to the data i.e., files stored on a disk.

Read: The process of non-destructive (e.g., non-recording) data retrieval from the disk.

Read/Write Head: See "Head".

Real-time: Immediate processing of input or notification of status.

Rebuilding: Re-creation of data lost when a drive fails. Also called Reconstruction.

Redundant: Duplicated, as in a redundant cabling system that provides a duplicate cabling route in case the first one goes down.

RISC: Reduced instruction set computer. A computer processing architecture that requires fewer instructions to run applications, thus increasing processing speed.

RLL: Run Length Limited. A data encoding scheme incorporated into ESDI, SCSI, and IPI-2 drives. Specifies the number of flux changes that can be written in a given amount of disk area. Allows 50% more flux changes in a given disks space than EFM.

ROM: Read-Only Memory.

Rotational latency: The delay time from when a disk drive's read/write head is on-track and when the requested data rotates under it.

Router: An electronic device that connects two or more networks and routes incoming data packets to the appropriate network.

RPM: Revolutions Per Minute. How many times a disk drive's platters make a complete revolution in a minute. A direct component of Rotational Latency time. (Divide by 60 to get Revolutions Per Second.)

Rotational Latency: The time it takes for the desired disk sector to be put into position under the read/write head, which must be done before data can be read from or written to the disk. For example, 5,400 RPM disks take II, 1ms to complete a full revolution and the average latency is 5.56 ms.


SAF-TE: SCSI Accessed Fault-Tolerant Enclosure. A bus management interface that allows an enclosure to be monitored and managed through the SCSI bus by a SAF-TE compliant controller. This is similar to the SES protocol.

SAN: High-speed network that establishes direct connections between storage systems and servers or clients using interconnect technologies such as routers, hubs, switches and gateways.

Scalable: The ability of a product or network to accommodate growth.

Scattering: When Spanning is used under NetWare entire blocks of data are scattered among the drives, which have been spanned into a single volume.

SCSI: Small Computer System Interface. An interface that serves as an expansion bus that can be used to connect hard disk drives, tape drive, and other hardware components.

SE: Single Ended. A SCSI protocol logic scheme.

Sector: A track unit that is physically read from or written to. Usually one sector is configured to 512 bytes.

Seek time: The time it takes for the disk heads to get into position over the appropriate track. Average Seek Time is derived from a number of measured random seeks and the head setting time, which is the time required for the heads' mechanical and electrical components to stabilize after positioning on the target cylinder.

Separator (Data): Convert the analog serials from the data head into digital pulses (i.e., either an I or an O) for transfer to the host PC bus.

Sequential: In order, in sequence. Refers to the data (i.e., files) stored on a drive.

Serial: One at a time. During data transfer, as when hits are transferred one-at-a-time in a "straight line" AKA "Sequential".

Serial device: A device that can only transmit data in a serial fashion (i.e., one bit at a time). Tape drives are serial devices. For example, to access a particular file, all files stored on the tape before it (or after it, if searching in reverse) must first be passed over.

Server: A computer that stores application and data files for all workstations on a network. Also referred to as a file server.

Servo: A drive motor wherein an electromagnetic coil built is around stationary magnets. The coil to which the head arm is attached, moves between the permanent magnets when current is applied. The position in this type of voice-coil drive is feedback-controlled, which results in highly reliable head positioning over narrow, closely spaced tracks.

SES Controller: SES stands for ‘SCSI Enclosure Services’. SES is a bus management protocol that can be implemented into a controller and enclosure. When used, it allows the controller to perform basic management functions on the enclosure when no user is available such as monitor power supply output and enclosure temperature. SES is currently used for Fibre Channel products and is similar to SAF-TE, which is used for SCSI products.

SLED: Single Large Expensive Disk. The opposite of RAID.

Soft error: Occurs when data is unable to be either read from or written to an area of the disk. A soft error may be recoverable (i.e., if not read the first time, may be read in a second attempt). A soft error that cannot be recovered from becomes a hard error (See "Hard Error").

Spanning: How NetWare combines multiple physical drives into a single volume, a process vaguely resembling RAID 1. It is software driven rather than hardware driven.

Spindle: Mechanism inside a hard disk drive that moves the heads into place. The axle on which a disk turns.

Spindle motor: The motor that spins the platters. The RPMs specification derives from how fast this motor is in the discs.

SSA: Serial Storage Architecture. A high-speed method of connecting disk, tape and CD-ROM drives, printers, scanners and other devices to a computer.

Streaming: Transfer of data without stopping in for data access, media adjust, etc.

Striping: The process of equally dividing a single logical block into multiple physical blocks on multiple disk drives.

Sustained mode: The measured transfer rate of a given device during normal operation.

Switch: A network traffic monitoring device that controls the flow of traffic between multiple network nodes.

System integrator: An individual or company that combines various components and programs into a functioning system, customized for a particular customer's needs.

Synchronous: Used in SCSI data transmission. Transfers data using clock pulses for timing and ACK/REQ handshaking. Is currently capable of transferring data up to 10MB per second.


Target: A SCSI device that performs an operation requested by an initiator.

TCQ: Tag Command Queuing. A feature introduced in the SCSI-2 specification that permits each initiator to issue commands accompanied by instructions for how the target should handle the command. The initiator can either request the command to be executed at the first available opportunity, in the order in which the command was received, or at a time deemed appropriate by the target.

Telco: Abbreviation for "telecommunications company".

Terabyte: Approximately one trillion bytes, 1,024 gigabytes.

Thermal Recalibration: Necessary to disk drive operation. The media expands and contracts with heat and cold temperature changes. Thermal recalibration occurs as often as necessary in order for the drive to ensure that data is written to where it is supposed to be. Without this very important feature, write errors would occur in unacceptable numbers. Over recalibrating degrades performance, as it requires the full attention of the drive to perform.

Throughput: Measures the number of service requests on the I/O channel per unit of time.

TPI: Tracks Per Inch, or track density.

Topology: Geometric arrangement of nodes and cable links in a local area network. May be either centralized or decentralized.

Track: Recording path shaped like concentric rings on a cylinder. Formed when the magnetic media moves a 5t at the read/write head.

Transfer Rate: How many MB of data can be transferred from the read/write heads to the disk controller in a single second (not to be confused with drive-to-host adapter transfer rate). Measured in megabytes per second MB/Sec.

Transfer Rate - Burst: The amount of data that can be transferred from a SCSI drive's buffer to the SCSI HBA (RAM-to-RAM transfer).

Transfer Rate - Maximum: How many theoretical megabytes can be transferred in a second. SCSI-1 and SCSI-2 are both 5MB/Sec: SCSI-2 Fast is 10MB/Sec; Maximum: SCSI-2 Wide is 20MB/Sec. AKA "External Transfer Rate".

Transfer Rate Sustained: How many actual megabytes can be transferred in a second, averaged over a period of time, usually an hour. This is the true measurement of a system's overall performance.

Turnkey: A product or system that can be plugged in, turned on, and operated with little or no additional configuring.


Unix: An operating system that supports multitasking and is ideally suited to multi-user applications (such as networks).

Ultra-2 Wide SCSI: An LVD SCSI based device interface. This protocol has an 80MB/s-transfer rate. The predecessor to this protocol called Ultra Wide SCSI is a SE protocol that only runs half the speed at 40MB/s.


VAR: Value-Added Reseller. A business that repackages and improves hardware manufactured by an original equipment manufacturer.


WAN: Wide-Area Network. A network that uses high-speed, long-distance communications technology (e.g., phone lines and satellites) to connect computers over long distances.

Web cache: A Web cache fills requests from the Web server, stores the requested information locally, and sends the information to the client. The next time the Web cache gets a request for the same information, it simply returns the locally cached data instead of searching over the Internet, thus, reducing Internet traffic and response time.

Web site: A location on the World Wide Web that is owned and managed by an individual, company or organization. Usually contains a home page and additional pages that include information provided by the site's owner, and may include links to other relevant sites.

World Wide Web: A global hypertext system operating on the Internet that enables electronic communication of text, graphics, audio and video.

WORM: Write-Once, Read Many. Optical drive that stores data to a removable disk cartridge such that the data cannot be written over once it has been stored. Requires special software to handle.

Write: The process of recording data on a disk surface via electromagnetic head.


XOR engine: A process or set of instructions that calculates data bit relationships in a RAID subsystem.






























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