GLOSSARY of RAID TERMINOLOGY

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

-- A -- 

Active termination - A type of terminator used in current SCSI channel setups, which require a specific way of termination. Active = automatic recognition of need for termination. 

Active/active - Synonym for dual active controllers 

Active/passive - Synonym for hot standby 

ANSI - American National Standards Institute, a standards organization. 

Applet - Small application programs" that are usually built into an operating system or a larger application program. The built-in writing and drawing programs that come with Windows are sometimes called "applets." Applications that run in a web browser are often Java-based applets. 

Application Server - An application program is designed to perform a specific function directly for the user or, in some cases, for another application program. Examples of applications include word processors, database programs, Web browsers, development tools, drawing, paint, and image editing programs, and communication programs. An application server is a centralized computer that holds and distributes these applications to users. 

Array - See storage array, disk array, tape array 

ASIC - Application-Specific Integrated Circuit 

Asynchronous data transfer - Data transfer not synchronized to a set timing interval. Asynchronous devices must wait for a signal from the receiving device after each byte. 

Automatic switchover - See failover 

Auto Swap - The functional substitution of a replacement unit in a disk system for a defective one, where the substitution is performed by the disk system itself while it continues to perform its normal function without human intervention (compare with Hot Swap). 

-- B -- (TOP) 

Background Initialization - The process whereby the initialization process of a disk array takes place in the background. Use of background initialization allows use of a disk array within minutes instead of several hours. Also known as Immediate RAID Availability. 

Base - Refers to baseband signaling, which means that only Ethernet signals are carried on the medium.

100Base T - In 100 Mbps (megabits per second) Ethernet (known as Fast Ethernet), there are three types of physical wiring that can carry signals: 100BASE-T4 (four pairs of telephone twisted pair wire) 100BASE-TX (two pairs of data grade twisted-pair wire) 100BASE-FX (a two-strand fiber optic cable) This designation is an IEEE shorthand identifier. The "100" in the media type designation refers to the transmission speed of 100 Mbps. The "BASE" refers to baseband signaling, which means that only Ethernet signals are carried on the medium. The "T4," "TX," and "FX" refer to the physical medium that carries the signal. (Through repeaters, media segments of different physical types can be used in the same system.) 

10Base T - The most widely installed Ethernet local area networks (LANs) use ordinary telephone twisted-pair wire. When used on Ethernet, this carrier medium is known as 10BASE-T. 10BASE-T supports Ethernet's 10 Mbps transmission speed. In addition to 10BASE-T, 10 megabit Ethernet can be implemented with these media types: 10BASE-2 (Thinwire coaxial cable with a maximum segment length of 185 meters) 10BASE-5 (Thickwire coaxial cable with a maximum segment length of 500 meters) 10BASE-F (Fiber optic cable) The "10" in the media type designation refers to the transmission speed of 10 Mbps. The "BASE" refers to baseband signaling, which means that only Ethernet signals are carried on the medium. The "T" represents twisted-pair; the "F" represents fiber optic cable; and the "2" and "5" refer to the coaxial cable segment length (the 185 meter length has been rounded up to "2" for 200).

BBU - Battery Backup Unit. A BBU provides a battery backup for a RAID controller’s cache. In the event of a power failure, the BBU can save data from the cache to the disk array. 

Benchmarks - A set of conditions or criteria against which a product or system is measured. Computer trade magazine laboratories frequently test and compare several new computers or computer devices against the same set of application programs, user interactions, and contextual situations. The total context against which all products are measured and compared is referred to as the benchmark. Programs can be specially designed to provide measurements for a particular operating system or application. 

Berkeley RAID Levels - A family of disk array protection and mapping techniques described by Garth Gibson, Randy Katz, and David Patterson in papers written while they were performing research into I/OP systems at the University of California at Berkeley. There are six Berkeley RAID levels, usually referred to by the names RAID Level 1 through RAID Level 6. See RAID levels. 

BIOS - Basic Input/Output System, software that determines what a computer can do without accessing programs. The BIOS contains all the code required to control the keyboard, screen, drives, serial communications, and other functions. Usually the BIOS is built into a ROM chip installed on the motherboard so that the BIOS will always be available and not be affected by failure of a disk. Sometimes the BIOS is recorded on a flash memory chip. 

BIOS Config Utility - BIOS-Based Confirmation Utility; utility program sequences that are used, upon powerup, for configuring various hardware elements in a system. 

Booting (or Bootstrapping) - The loading of operating system code from a disk or other storage device into a computer’s memory 

Bridge RAID Controller - A SCSI device appearing as a single ID on the SCSI bus, but which bridges to multiple devices, typically used to control external RAID subsystems (compare to Internal PCI-based RAID subsystems; see Internal RAID Controller.). A bridge RAID controller is often referred to as an external RAID controller. 

Burst data rate - The speed at which a specific amount of data sent or received in one intermittent operation (compare to Sustained Data Transfer Rate.) 

Bus - The channel through which data is transferred. There are two types, Serial and Parallel. 

-- C -- (TOP) 

Cables - The physical wires (copper or fibre optic) over which electrical signals are transmitted. Cables are used to connect peripherals (such as disk arrays) to computers and servers or to connect peripherals or components to each other. 

Cache - Controller memory used to speed up data transfer to and from a disk. 

Cache Flush - Refers to an operation where all unwritten blocks in a Write-Back Cache are written to the target disk. This operation is necessary before powering down the system. 

Cache Line Size - See Segment Size

Caching - Caching allows data to be stored in a pre-designated area of a disk or RAM. Caching is used to speed up the operation of RAID systems, disk drives, computers and servers, or other peripheral devices. 

CD-ROM - Compact Disk Read Only Memory. A removable read-only storage device similar to an audio compact laser disk, and holding up to 640M of information. 

Channel - Any path used for the transfer of data and control information between storage devices and a storage controller or I/O adapter. Also refers to one SCSI bus on a disk array controller. Each disk array controller provides at least one channel. 

CISC - Complex Instruction Set Computing, a general purpose CPU architecture. 

Cluster - A cluster is a group of terminals or workstations attached to a common control unit or server or a group of several servers that share work and may be able to back each other up if one server fails. 

CMOS - Complementary Metal Oxide Semiconductor, a chip that packs many components into a very small semiconductor. 

Cold Swap - The physical substitution of a replacement unit in a storage system for a defective one, where power is removed from the storage system in order to perform the substitution and human intervention is required (compare to Hot Swap and AutoSwap). 

Consistency Check - Refers to a process where the integrity of redundant data is verified. For example, a consistency check of a mirrored drive will make sure that the data on both drives of the mirrored pair is exactly the same. For RAID Level 5 redundancy, a consistency "disconnect" is a function that allows a target SCSI device (typically a disk drive that received a request to perform a relatively long I/O operation) to release the SCSI bus so that the controller can send commands to other devices. When the operation is complete and the SCSI bus is needed by the disconnected target again, it is "reconnected." 

Controller - An adapter card, RAID controller, or other module that interprets signals between a host and a peripheral device. 

CPU - Central Processing Unit, the circuitry that performs the basic instructions to drive the computer. 

Critical - A Logical Drive is in a "critical" state if it has been configured at RAID level 1, 3, 5, or 0+1; and one (and only one) of its SCSI drives is not "online." A logical drive is considered "critical" because any failure of another of its SCSI drives may result in a loss of data. Note: I/O operation can only be performed with system drives that are online or critical. 

-- D -- (TOP) 

DACCF - Disk Array controller Configuration File, a diskette-based configuration utility (similar in function to BCU) for RAID configuration. 

DAC960 Series - A family of RAID controllers from Mylex Corporation. The DAC960 series includes both internal PCI-based RAID controllers and external RAID controllers. The DAC960 series controllers all use Intel’s® i960 microprocessor. DAC is an acronymfor Disk Array Controller. 

Data Mining - The analysis of data for relationships that have not previously been discovered. For an example, performing a deeper analysis of buying records may indicate seasonal preferences that have not been considered. 

Data Transfer 

Rate - The amount of data per unit time moved through a channel or I/O bus in the course of execution of an I/O load, usually expressed in Mbytes/second. 

DB-9 - A type of connector, used for serial interfaces. 

Degraded Mode - A RAID mode used when a component drive has failed. 

Desktop PC - Standard Intel-processor based personal computer used as an individual station (not as a server). 

Device Driver - A software program that controls a particular type of device attached to a computer, such as a RAID subsystem, printer, display, CD-ROM, disk drive, etc. 

Differential - A protocol that transmits through a current loop rather than by changes in voltage, reducing the susceptibility to electrical interference. 

Disk - A non-volatile, randomly addressable, re-writable data storage device, including rotating magnetic and optical disks as well as solid-state disks or other electronic storage elements. 

Disk Array - A collection of disks from one or more commonly accessible disk systems. Disk arrays, also known as RAID, allows for disk drives to be used jointly to improve fault tolerance, performance, or both. Disk arrays are commonly used on severs and are becoming more popular on desktops and workstations. 

Disk Drive - A device for the electronic digital storage of information. 

Disk Failure Detection - A RAID controller automatically detects SCSI disk failures. A monitoring process running on the controller checks, among other things, elapsed time on all commands issued to disks. A time-out causes the disk to be "reset" and the command to be retried. If the command times out again, the disk could be "killed" (taken "offline") by the controller (its state changed to "dead"). Some controllers also monitor SCSI bus parity errors and other potential problems. Any disk with too many errors will also be "killed." 

Disk Media Error Management - Some controllers transparently manage SCSI disk media errors. Disks are programmed to report errors, even ECC-recoverable errors. If ECC EDO RAM is installed, the controller will correct ECC errors. When a disk reports a media error during a read, the controller reads the data from the mirror (RAID 1 or 0+1), or computes the data from the other blocks (RAID 3, RAID 5), and writes the data back to the disk that encountered the error. If the write fails (media error on write), the controller issues a "reassign" command to the disk, and then writes the data to a new location. Since the problem has been resolved, no error is reported to the system. When a disk reports a media error during a write, the controller issues a "reassign" command to the disk, and writes the data out to a new location on the disk. 

Disk System - A storage system capable of supporting only disks. 

Drive Groups (or Drive Packs) - A group of individual disk drives (preferably identical) that are logically tied to each other and are addressed as a single unit. In some cases this may be called a drive "pack" when referring to just the physical devices. Up to eight (8) drives can be configured together as one drive group. All the physical devices in a drive group should have the same size, otherwise each of the disks in the group will effectively have the capacity of the smallest member. The total size of the drive group will be the size of the smallest disk in the group multiplied by the number of disks in the group. For example, if you have 4 disks of 400MB each, and 1 disk of 200MB in a pack, the effective capacity available for use is only 1000MB (5x200), not 1800MB. 

Drive Traveling - In the process of replacing a failed disk, disk traveling occurs when the drives are placed in a different order than the original one. 

Drivers - A software routine which receives I/O requests from higher levels within the operating system and converts those request to the protocol required by a specific hardware device. 

Dual active - A pair of components, such as storage controllers in a failure tolerant storage system that share a task or set of tasks when both are functioning normally. When one component of the pair fails, the other takes the entire load. Dual active controllers (also called active-active controllers) are connected to the same set of devices and provide a combination of higher I/O performance and greater failure tolerance than a single controller. 

-- E -- (TOP) 

ECC - Error Correcting Code. A method of generating redundant information which can be used to detect and correct errors in stored or transmitted data. 

EDO - Extended Data Output, a type of random access memory (RAM) chip designed to improve the time to read from memory on faster microprocessors such as the Intel Pentium. 

EEPROM - Electrically Erasable PROM 

EISA - Extended Industry Standard Architecture. A bus standard for PCs extending the ISA architecture to 32 bits and allowing more than one CPU to share the bus. 

Embedded 

Storage Controller - An intelligent storage controller that mounts in a host computer’s housing and attaches directly to a host’s memory bus with no intervening I/O adapter or I/O bus. 

EPROM - Erasable Programmable Read Only Memory which can be erased and re-used. 

Ethernet - The most widely-installed local area network technology. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. 

External RAID Controller - A RAID controller that is in its own enclosure, rather than incorporated into a PC or server. External RAID controllers are often referred to as a bridge controller. 

Extranet - A private network that uses the Internet protocols and the public telecommunication system to securely share part of a business's information or operations with suppliers, vendors, partners, customers, or other businesses. An extranet can be viewed as part of a company's intranet that is extended to users outside the company. 

-- F -- (TOP) 

Failure - A detectable physical change in hardware, repaired by the replacement of the component. 

Failback - The restoration of a failed system component’s share of a load to a replacement component. 

Failover - A mode of operation for failure tolerant systems in which a component has failed and its function has been assumed by a redundant component. 

Fast Ethernet - A local area network (LAN) transmission standard that provides a data rate of 100 megabits per second (referred to as "100BASE-T10"). 

Fast SCSI - Devices using timings up to 10MHz. (compare to Fast/Wide SCSI, Ultra SCSI and Ultra2 SCSI). 

Fast/Wide SCSI - SCSI devices using timings up to 20 MHz (compare to Fast SCSI, Ultra SCSI and Ultra2 SCSI). 

FAT, FAT 32 - File Allocation Table maintained by an operating system on a hard disk that provides a map of the clusters (the basic unit of logical storage on a hard disk) that a file has been stored in. FAT 32 supports 32-bit processors 

Fault tolerance (Failure tolerance) - The ability of a system to continue to perform its function even when one of its components has failed. For a system to be fault tolerant requires redundancy in disk drives, power supplies, adapters, controllers and cabling. 

Fibre Channel - A technology for transmitting data between computer devices at a data rate of up to 1 Gbps (one billion bits per second), especially suited for connecting computer servers to shared storage devices and for interconnecting storage controllers and drives. Fibre Channel is expected to replace the Small System Computer Interface (SCSI) as the transmission interface between servers and clustered storage devices. It is also more flexible; devices can be as far as ten kilometers (about six miles) apart. The longer distance requires optical fiber as the physical medium. However, Fibre Channel also works using coaxial cable and ordinary telephone twisted pair. 

File Server - A computer shared by users over a Local Area Network (LAN). A file server provides access to files by the users on a LAN. Other types of servers perform application services (for example, mail servers, web servers) rather than just store files. 

Flash ROM - A memory on the adapter containing software that can be reprogrammed without removing it from the board. 

Format - A pre-established layout for data. Programs accept data as input in a certain format, process it, and provide it as output in the same or another format. All data is stored in some format with the expectation that it will be processed by a program that knows how to handle that format. 

-- G -- (TOP) 

Gigabit - 1,000,000,000 (10 9 ) bits 

Gigabyte - 1, 000,000,000 (10 9 ) bytes 

Global Array Manager (GAM) - A software GUI package that allows a system administrator to configure, monitor, and manage network RAID storage from anywhere in the world with the Mylex Global Array Manager™ (GAM). Communicate critical notification via e-mail, fax, pager, SNMP or the launching of an application. GAM is everything needed to manage Mylex RAID Controllers, SCSI Host Adapters, and external RAID controllers. 

Graphical User Interface - A software interface that interacts with the user through color graphics and a mouse. 

GUI - See Graphical User Interface. 

-- H -- (TOP) 

Hard Disk Drive - The main data storage unit of a computer. 

HDD - See Hard Disk Drive 

HDM - Hardware Driver Module, a driver set required for SCSI adapters for use in an I2O environment. 

Host - Any computer system to which disks are attached and accessible for data storage and I/O. 

Host Bus Adapter (HBA) - An I/O adapter that connects a host I/O bus to the host’s storage memory system. 

Host I/O Bus - An I/O bus used to connect a host computer to storage systems or storage devices. 

Hot Plug - The operation of adding or removing a device from a bus while transactions involving other devices are occurring over the bus. 

Hot Replacement of Disks (Hot Swap) -The design of some controllers allows for the replacement of failed hard disk drives without interruption of system service. In the event of a SCSI drive failure on a properly configured system (where the data redundancy features of the controller are used), system service continues without interruption. A message is generated by the system to alert the system operator. When a replacement drive becomes available, the system operator can remove the failed disk drive, install a new disk drive, and instruct the controller to "rebuild" the data on the new drive, all without interrupting system operations. Once the rebuild is complete, the controller will be brought back into a fault tolerant state. 

Hot Spare - See Standby Replacement of Disks 

Hot Standby - A redundant component is a fault tolerant storage system that has power applied and is ready to operate, but which does not perform its task as long as the primary component for which it is standing by is functioning properly. 

Hot Swap - The substitution of a replacement unit in a storage system for a defective unit, executed while the system is performing its normal function and requiring human intervention. (Compare with AutoSwap and Cold Swap). 

-- I -- (TOP) 

I/O - Input/output, the transmission of information between an external source and the computer. 

I/O bottleneck - Any resource in the I/O path whose performance limits the performance of a storage or I/O system. 

I/O Bus - Any path used for the transfer of data and control information between I/O adapters and storage controllers or storage devices. 

I/O intensive - An application whose performance depends strongly on the performance of the I/O system. 

I/Os per second - Number of I/O transactions per second 

I2O - Intelligent Input/Output 

Immediate RAID Availability - See Background Initialization 

Internal RAID Controller - A controller circuit board that resides inside a computer or server. An internal RAID controller resides on a bus, such as the PCI bus. 

Internet - A worldwide system of computer, originally conceived by the U.S. government to construct a network that would continue to function even if a large portion of it were destroyed, for example, in the event of war. The Internet is now a public, cooperative, and self-sustaining facility accessible to tens of millions of users worldwide. 

Intranet - A network contained within an enterprise, the main purpose of which is usually to share company information and computing resources among employees. An intranet can also be used to facilitate working in groups and for teleconferences. 

ISA - A standard bus (computer interconnection) architecture associated with the IBM AT motherboard. It allows 16 bits at a time to flow between the motherboard circuitry and an expansion slot card and its associated device(s). 

ISM - Integrated Software Module, used in I2O. 

-- J -- (TOP) 

Java - A programming language expressly designed for use in the distributed environment of the Internet and enforces a completely object-oriented view of programming. Java can be used to create complete applications that may run on a single computer or be distributed among servers and clients in a network. It can also be used to build small application modules or applets for use as part of a Web page. 

JBOD - Just A Bunch of Disks (Drives), a number of disk drives, usually in an enclosure. JBOD implies that the disks are not RAIDed and function independently. 

-- K -- (TOP) 

Kbyte - approximately a thousand bytes (actually, 2 to the 10th power, expressed in decimal as 1,024). 

-- L -- (TOP) 

LAN - Local Area Network, a network of interconnected workstations sharing the resources of a single processor or server, typically within the area of a small office building. 

Latency - 1. The time between the making of an I/O request and completion of the request’s execution. 2. Short for rotational latency, the time between the completion of a seek and the instant of arrival of the first block of data to be transferred at the disk’s read/write head. 

Logical Drive States - The state of a logical (system) drive can be either ONLINE, CRITICAL, or OFFLINE. Notice that the same term "online" is used for both physical and logical drives. 

Logical Drives - See System Drives 

LVD (Low Voltage Differential) - Low Voltage Differential is a form of SCSI signalling that was introduced with Ultra2 SCSI. Low Voltage Differential allows cable lengths of up to 12 meters (approximately 39 feet) with up to 15 devices. The devices need to be Ultra2 SCSI LVD devices in order to take advantage of the LVD signalling. LVD uses data high and data low signal lines to increase transmission distances over those of single-ended (conventional SCSI signaling). LVD differs from conventional differential signaling. LVDs advantages are that it consumes less power and it can sense single-ended devices on the bus and revert to single-ended signalling. 

-- M -- (TOP) 

Megabit - A million bits, used as a common unit of measure relative to time in seconds as an expression of a transmission technology's bandwidth or information flow capacity. Megabits per second (Mbps) is a frequent measure of bandwidth on a transmission medium. 

Megabyte (Mbyte) - 1,000,000 (10 6 ) bytes 

Mirrored Cache - A cache memory that has duplicate data from another controller. In the event of failure of the original controller, the second controller can take the cached data and place it on the disk array. 

Mirroring - Refers to the complete duplication of data on one disk drive to another disk drive, this duplication occurring simultaneously with each read and write operation. Each disk will be the mirror image of the other. This is also known as RAID level 1.

MTBF - Mean Time Between Failures, the average time from start of use to failure in a large population of identical systems, computers, or devices. 

MTDL - Mean Time until Data Loss, the average time from startup until a component failure causes a permanent loss of user data in a large population of disk arrays. 

-- N -- (TOP) 

Narrow SCSI - 8-bit wide standard SCSI. 

NFS - Network File System, a client/server application that lets a computer user view and optionally store and update files on a remote computer as though they were on the user's own computer. 

NOS - Network Operating System 

NVRAM - Non-Volatile Random Access Memory, a memory unit equipped with a battery so that the data stays intact even after the main power had been switched off.

-- O -- (TOP) 

Offline - A Logical Drive is in an "offline" state if no data can be read from it or written to it. System commands issued to offline logical drives are returned with an error status: no operations can be performed on offline logical drives. 

OLTP - Online Transaction Processing 

Online - A Logical Drive is in an "online" state if all of its participating SCSI drives are "online" and operational. 

OS - Operating System, software that manages the system resources and provides the operating environment for application programs. 

-- P -- (TOP) 

Pack - See Drive Groups (or Drive Packs) 

Parity - See "Rotated XOR Redundancy." A method of providing complete data redundancy while requiring only a fraction of the storage capacity of mirroring. Partitioning - The presentation of the full usable storage capacity of a disk or array of disks to an operating environment in the form of several virtual disks whose aggregate capacity approximates that of the underlying disk or array. 

PCB - Printed Circuit Board 

PCI - Peripheral Component Interconnect - This is a high-performance, backplane interface, expansion slot architecture that is found on PCs, Macintoshes, and UNIX workstations. PCI is a standardized architecture that provides a high-speed data path between peripherals and the CPU. PCI cards are portable across hardware platforms with the help of various software drivers. 

PCI Hot Plug - An essential feature in newer PCI-based PCs which allows for the printed circuit board to be replaced without powering down the entire system.  

Peripheral - Any computer device that is not part of the essential computer (the processor, memory, and data paths) but is situated relatively close by. 

Platform - An underlying computer system on which application programs can run. A platform consists of an operating system, the computer system's coordinating program, and a microprocessor, the microchip in the computer that performs logic operations and manages data movement in the computer. 

Plug and Play - The ability to install peripheral cards or other devices without requiring manual configuration by the user. 

PROM - Read-only memory that can, with provided instructions, be reprogrammed by a user. 

Protocol - The special set of rules for communicating that the end points in a telecommunication connection use when they send signals back and forth. 

-- Q -- (TOP) 

Queue - A line of things, commands, or data waiting to be handled, usually in sequential order starting at the beginning or top of the line or sequence. 

-- R -- (TOP) 

RAID - RAID stands for Redundant Array of Independent Disks. RAID controllers implement this technology to connect up to 15 SCSI devices per channel. Several different forms of RAID implementation have been defined. Each form is usually referred to as a "RAID level." All the RAID levels supported by DAC960 Series controllers are shown below. The appropriate RAID level for a system is selected by the system manager or integrator. This decision will be based on which of the following are to be emphasized: Disk Capacity Data Availability (redundancy or fault tolerance) Disk Performance 

RAID Adapters - Low cost RAID controllers that also use SCSI channels on the motherboard. 

RAID Advisory board (RAB) - An association of companies whose primary intention is in standardization of RAID in storage systems. 

RAID Controller - See RAID Adapter 

RAID Levels - 

Level 0: Block "striping" across multiple drives is provided, yielding higher performance than is possible with individual drives. This level does not provide any redundancy.
Level 1: Drives are paired and mirrored. All data is 100 percent duplicated on a drive of equivalent size.
Level 3: Data is "striped" across several physical drives. Maintains parity information which can be used for data recovery.
Level 5: Data is "striped" across several physical drives. For data redundancy, drives are encoded with rotated XOR redundancy.
Level 0+1: Combines RAID 0 striping and RAID 1 mirroring. This level provides redundancy through mirroring.
JBOD: Sometimes referred to as "Just a Bunch of Drives." Each drive is operated independently like a normal disk controller, or drives may be spanned and seen as a single drive. This level does not provide data redundancy.
Level 10: Combines RAID 0 striping and RAID 1 mirroring spanned across multiple drive groups (super drive group). This level provides redundancy through mirroring.
Level 30: Data is "striped" across multiple drive groups (super drive group). Maintains parity information which can be used for data recovery.
Level 50: Data is "striped" across multiple drive groups (super drive group). For data redundancy, drives are encoded with rotated XOR redundancy. Note: The host operating system drivers and software utilities remain unchanged regardless of the level of RAID installed. The controller makes the physical configuration and RAID level implementation 

RAID Migration - A features in RAID subsystems that allows for changing a RAID level to another level without powering down the system. 

RAM - Random Access Memory, the "built-in" readable and writable data storage that comes with (or can be added to) a computer. 

Recovery - The process of reconstructing data from a failed disk using data from other drives. 

Redundancy - The inclusion of extra components of a given type in a system (beyond those required by the system to carry our its functions.) 

Replacement Table - A replacement table contains information regarding which SCSI devices have been replaced by others through standby replacement. 

RISC - Reduced Instruction Set Computing, an architecture for an application-specific processor. 

RJ-11, RJ-45 - RJ (registered jacks, sometimes described as RJ-XX), a series of telephone connection interfaces (receptacle and plug) that are registered with the U.S. Federal Communications Commission (FCC). The most common telephone jack is the RJ-11 jack, which can have six conductors but usually is implemented with four. The RJ-11 jack is likely to be the jack that your household or office phones are plugged into from the ordinary "untwisted" wire (sometimes called "gray satin" or "flat wire") that people are most familiar with. The RJ-45 is a single-line jack for digital transmission over ordinary phone wire, either untwisted or twisted. The interface has eight pins or positions. 

ROM - Built-in computer memory containing data that normally can only be read, not written to. ROM contains the programming that allows a computer to be "booted up" each time you turn it on. Unlike a computer's random access memory (RAM), the data in ROM is not lost when the computer power is turned off. The ROM is sustained by a small long-life battery in your computer. 

Rotated XOR Redundancy - This term (also known as "parity") refers to a method of providing complete data redundancy while requiring only a fraction of the storage capacity of mirroring. In a system configured under RAID 3 or RAID 5 (which require at least three SCSI drives), all data and parity blocks are divided between the drives in such a way that if any single drive is removed (or fails), the data on it can be reconstructed using the data on the remaining drives. (XOR refers to the Boolean "Exclusive-OR" operator.) In any RAID 3 or RAID 5 array, the capacity allocated to redundancy is the equivalent of one drive. 

RS 232C - Recommended Standard 232 Revision C, a long-established standard that describes the physical interface and protocol for relatively low-speed serial data communication between computers and related devices. RS-232C is the interface that a computer uses to talk to and exchange data with your modem and other serial devices. It is asynchronous 25-pin connector. 

RS 422/423 - Recommended Standard 422/423, a 32-pin serial connector that can be used for both synchronous or asynchronous transmission of data. The 422 is differential; the 423 is single line. 

RTC - Real Time Controller 

-- S -- (TOP) 

SAF-TE - SCSI Accessed Fault-Tolerant Enclosure, an "open" specification designed to provide a comprehensive standardized method to monitor and report status information on the condition of disk drives, power supplies and cooling systems used in high availability LAN servers and storage subsystems. The specification is independent of hardware I/O cabling, operating systems, server platforms, and RAID implementation because the enclosure itself is treated as simply another device on the SCSI bus. The SAF-TE specification has been endorsed by many other leading server, storage and RAID controller manufacturers worldwide. Products compliant with the SAF-TE specification will reduce the cost of managing storage enclosures, making it easier for a LAN administrator to obtain base-level fault-tolerant alert notification and status information. 

SCA - A type of interface designed to provide a standard connection for systems using hot swappable drives. SCA inters drives connect to a SCSI backplane that provides power and configuration settings such as SCSI IDF and termination of the SCSI bus. 

SCSI - SCSI is an acronym for "Small Computer Storage Interface," although it is often used in large systems. The SCSI interface supports disk drives, tape drives, and CDROM drives. Narrow SCSI supports up to 7 devices on a channel, and Wide SCSI, Ultra SCSI, and Ultra2 SCSI support up to 15 devices on a channel. 

SCSI Adapters - Storage controllers for managing SCSI devices 

SCSI Drive - A disk drive equipped with a SCSI interface (sometimes referred to as a SCSI Disk). Each disk drive will be assigned a SCSI address (or SCSI ID), which is a number from 0 to 7 (0 to 15 under wide or Ultra SCSI). The SCSI address uniquely identifies the drive on the SCSI bus or channel. 

SCSI Drive States - Refers to a SCSI drive’s current operational status. At any given time, a SCSI drive can be in one of five states: READY, ONLINE, STANDBY, DEAD, or REBUILD. The controller stores the state of the attached SCSI drives in its non-volatile memory. This information is retained even after power-off. Hence, if a SCSI disk is labeled DEAD in one session, it will stay in the DEAD state until a change is made either by using a system level utility or after a rebuild. Each of the states is described below: 

Ready: A SCSI disk drive is in a "ready" state if it is powered on and is available to be configured during the current session but remains unconfigured.
Online: A SCSI disk drive is in an "online" state if is powered on, has been defined as a member of a drive group; and is operating properly.
Standby: A SCSI disk drive is in a "standby" state if it is powered on, is able to operate properly, and was NOT defined as part of any drive group.
Dead: A SCSI disk drive is in a "dead" state if it is not present; or if it is present but not powered on; or if it failed to operate properly and was ‘killed’ by the controller. When the controller detects a failure on a disk, it "kills" that disk by changing its state to "dead." A SCSI drive in a dead state does not participate in any I/O activity. No commands are issued to dead drives.
Rebuild: A SCSI disk drive is in a "rebuild" state while it is in the process of being rebuilt. During this process, data is regenerated and written to the disk drive. This state is also referred to as ‘Write-Only’ (WRO). 

SCSI Interrupt Steering Logic (SISL) - This is the architecture that allows a RAID controller, such as AcceleRAID to implement RAID on a system board-embedded SCSI bus or a set of SCSI busses. 

SCSIVision - A set of utilities for ease of SCSI configuration and management in SCSI controllers and storage.

SDRAM - Synchronous Dynamic Random Access Memory, a new form of dynamic random access memory (DRAM) that can be coordinated or synchronized to the clock speed of the computer. 

Sector - The unit in which data is physically stored and protected against errors on a fixed-block architecture disk. 

Segment Size - The Segment Size function is set in conjunction with stripe size and represents the size of the data "chunk" that will be read or written at one time. Under DACCF, the segment size (also known as "cache line size") should be based on the stripe size you selected. 

Sequential I/O - A type of read and write operation where entire blocks of data are accessed one after another in sequence, as opposed to randomly. 

Server - A computer program that provides services to other computer programs in the same or other computers. The computer that a server program runs in is also frequently referred to as a server. 

SES - SCSI-3 Enclosure Services, a standard in the making for SCSI access to services within an enclosure containing on or more SCSI devices. 

Session - Refers to the period of time between any two consecutive system shutdowns. System shutdown may be either a power off/on, or a hardware reset. 

SIMM - Single In-Line Memory Module, RAM packed on a small circuit board with a defined edge connector. 

SISL - See "SCSI Interrupt Steering Logic." 

SMART - Self Monitoring Analysis and Reporting Technology, the industry standard reliability prediction indicator for both the IDE/ATA and SCSI hard disk drives. Hard disk drives with SMART offer early warning of some hard disk failures so critical data can be protected. 

SMB - Server Message Block protocol, a method for client applications in a computer to read and write to files on and to request services from server programs in a computer network. A client application can read, create, and update files on the remote server. It can also communicate with any server program that is set up to receive an SMB client request. 

SNMP - Simple Network Management Protocol, the protocol governing network management and the monitoring of network devices and their functions. 

Software Driver - Set of utilities and instructions for controllers to work in a specific operating system environment. 

Standard Disk Drive - This term refers to a hard disk drive with SCSI, IDE, or other interface, that is attached to the host system through a standard disk controller. 

Standby Replacement of Disks ("Hot Spare") - One of the most important features the RAID controller provides to achieve automatic, non-stop service with a high degree of fault-tolerance. The rebuild operation will be carried out by the controller automatically when a SCSI disk drive fails and both of the following conditions are true: A "standby" SCSI disk drive of identical size is found attached to the same controller; All of the system drives that are dependent on the failed disk are redundant system drives, e.g., RAID 1, RAID 3, RAID 5, RAID 0+1.  A "Standby" disk can be created in one of two ways: 1. When a user runs a config utility, all disks attached to the controller that are NOT configured into any drive group will be automatically labeled as "standby" drives. 2. A disk may also be added (attached at a later time) to a running system and labeled as standby by using the "DAC960 Software Kit" (see appropriate information from Mylex for DAC960 utilities for a particular operating system). During the automatic rebuild process, system activity continues as normal. System performance may degrade slightly during the rebuild process. To use the standby rebuild feature, you should always maintain a standby SCSI disk in your system. When a disk fails, the standby disk will automatically replace the failed drive and the data will be rebuilt. The system administrator can disconnect and remove the bad disk and replace it with a new disk. The administrator can then make this new disk a standby. The standby replacement table has a limit of 8 automatic replacements in any session (from power-on/reset to the next power-off/reset). When the limit of 8 is reached and a disk failure occurs, the standby replacement will occur but will not be recorded in the replacement table. To clear the ‘standby replacement’ table, reboot the system from a DOS bootable floppy, run the configuration utility and select the option ‘view/update configuration’ from the main menu. A red box labeled ‘Drive Remap List’ will be displayed. Selecting the box will allow you to continue. You should save the configuration without making any changes, and exit the configuration utility. This will clear the replacement table. You may now proceed to boot your system and continue normal operations. In normal use, the replacement table limit of 8 should not cause any problems. Assuming that a disk fails about once a year (drives we support generally come with a 5-year warranty), the system would run continuously for a minimum of 8 years before the table would need to be cleared.

Storage Area Networking (SAN) - A high-speed, open-standard scalable network of storage devices and servers providing accelerated data access 

Storage Device - A collective term for disks, tape transports, and other mechanisms capable of non-volatile data storage. 

Stripe Order - The order in which SCSI disk drives appear within a drive group. This order must be maintained, and is critical to the controller’s ability to ‘Rebuild’ failed drives. 

Stripe Size - The stripe size is defined as the size, in kilobytes (1024 bytes) of a single I/O operation. A stripe of data (data residing in actual physical disk sectors, which are logically ordered first to last) is divided over all disks in the drive group. 

Stripe Width - The number of striped SCSI drives within a drive group. 

Striping - Refers to the storing of a sequential block of incoming data across multiple SCSI drives in a group. For example, if there are 3 SCSI drives in a group, the data will be separated into blocks and block 1 of the data will be stored on SCSI drive 1, block 2 on SCSI drive 2, block 3 on SCSI drive 3, block 4 on SCSI drive 1, block 5 on SCSI drive 2 and so on. This storage method increases the disk system throughput by ensuring a balanced load among all drives. 

Sub-System Storage - A collection of disks providing data storage space to a system user. 

Sustained Data Transfer Rate - A rate of data transfer defined for continuous operation at a maximum speed level. 

Synchronous data transfer - Data transmission synchronized to a defined time interval; faster than asynchronous SCSI because there is no wait for acknowledgement of each byte from the receiving device (up to 20MHz). 

System Disk - The disk on which a system’s operating software is stored. 

System Drives - A system drive is equivalent to a "logical" drive. System drives are presented to the operating system as available disk drives, each with a capacity specified by the DAC960 Series controller. 

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Target ID - A target ID is the SCSI ID of a device attached to a DAC960 controller. Each SCSI channel can have up to 15 attached SCSI devices (target ID from 0 to 6, and 8 to 15). 

TCP-IP - Transmission Control Protocol/Internet Protocol, the basic communication language or protocol of the Internet. It can also be used as a communications protocol in intranets and in extranets. When set up with direct access to the Internet, a computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP. 

Terabyte - A measure of memory capacity, two to the 40th power or "roughly" (as a decimal number) a thousand billion bytes (that is, a thousand gigabytes). 

Termination - A method of matching transmission impedance of a bus to eliminate signal reflections from the physical ends of the bus. 

Thin file Server - A small server maximized for the sole purpose of serving files, as opposed to computing. 

Throughput - The number of I/O requests satisfied per unit time (usually per second.) 

Tpm-C, TPC-C - The Transaction Processing Performance Council (TPC) is a standards organization that measures transaction throughput of systems. One of their benchmarks is Tmp-C, which reflects price and performance metrics. TPC-C reflects new order transaction rate, a benchmark for transaction speed. Mylex products have won consistently high TPC-C results. 

Transfer Rate - The rate at which data moves between the host computer and storage, input, or output devices, usually expressed as a number of characters per second. 

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Ultra SCSI (Fast 20 SCSI) - A high performance SCSI protocol that has a bus speed of 20 MegaBytes per second in the Narrow SCSI configuration and 40 MB in the Wide SCSI (Fast 20 Wide SCSI) configuration. 

Ultra Wide SCSI - 16-bit wide Ultra SCSI (IS devices), double the speed of narrow SCSI. 

Ultra2 SCSI, or Fast 40 SCSI - A higher performance SCSI protocol than Ultra SCSI. Ultra2 SCSI has a bus speed of 40 MegaBytes per second in the Narrow SCSI configuration and 80 MegaBytes in the Wide SCSI (Wide Ultra2 SCSI) configuration. 

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WAN - Wide Area Network, a broader telecommunication structure as opposed to a local area network (LAN). A wide area network may be composed entirely of private structures, but the term seems to also connote the inclusion of public networks and all kinds of transmission media. 

Web - A number of computers, servers or networks linked together in a matrix. 

Wide SCSI - A SCSI protocol and signal definition providing 16-bit wide data path. 

WINTEL - A computer trade industry term for personal computers based on the Intel microprocessor and one of the Windows operating systems from Microsoft. The term "PC" has often been used for this purpose. 

Write Through Cache - Refers to a caching strategy whereby data is written to the SCSI drive before a completion status is returned to the host operating system. This caching strategy is considered more secure, since a power failure will be less likely to cause loss of data. However, a write through cache results in a slightly lower performance. 

Write-Back Cache - Refers to a caching strategy whereby write operations result in a completion signal being sent to the host operating system as soon as the cache (not the disk drive) receives the data to be written. The target SCSI drive will receive the data at a more appropriate time, in order to increase controller performance. An optional cache battery backup can be used to protect against data loss as a result of a power failure or system crash. 

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XOR - Exclusive "Or", a computer language function that generates parity in RAID systems. 

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Y2K - The Year 2000. Because some computer systems may have difficulty correctly handling dates after the millenium, manufacturers have been asked to check their hardware and software to see if their products will cause problems after the date change. See Mylex web site (www.mylex.com) for our Y2K statement. 

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with reference from Mylex Corporation