Adapter - SCSI Host Bus Adapter or HBA
ANSI - American National Standards Institute.
Array Group - A group of disk drives which appear to the computer as a single LSU. RAID-1 and RAID-5 Array Groups can be composed of any number of Hardware Arrays. RAID-0 Array Groups can be composed of any combination of individual disk drives.
ASPI - Advanced SCSI Programming Interface - A protocol used by some SCSI application programs to communicate with SCSI adapters under DOS, OS/2 and Netware.
Asynchronous - Data transfer protocol which is not synchronized to a set timing interval. Asynchronous SCSI data transmitting devices must wait after each byte for acknowledgement from the receiving device. Either device can take as long as it wishes to send or acknowledge data. Asynchronous SCSI has no defined maximum transfer rate but is typically limited to 1.5 to 3MHz.
BIOS - Basic Input Output System - a ROM-based collection of device drivers and system boot-up routines which is provided as an integral part of every PC computer. The BIOS provide enough intelligence to enable the computer to understand some simple keyboard commands and load the operating system from disk upon power-up. In order to do this, the BIOS must contain drivers for the CRT display, keyboard and disk controller.
Bridge Controller - A SCSI device which appears as a single ID on the SCSI bus, but which bridges to multiple devices. Each of these devices is assigned a Logical Unit Number or LUN. Bridge controllers are typically used to control remote RAID subsystems.
Build - The operation of initializing a Redundant Array by creating consistent redundant information. In the case of a RAID-1 array, one drive must be copied to the other. In the case of RAID-5 arrays, the parity information must be generated.
Burst - A term used to describe data words which are transmitted as a single group across a bus without interruption by another device.
Bus Mastering - A method of data transfer which allows data to be moved between a peripheral controller and system memory without interaction with the host CPU or a third party DMA controller. This technique allows the peripheral controller to take control of the system bus and move data at up to 10MB/s, 33MB/s or 132MB/s for ISA, EISA and PCI systems respectively.
Cache - A temporary fast storage area for data which would normally be accessed from a slower storage device. A cache management algorithm monitors the data access patterns and selects which data from the slower device is to be kept in the cache for quick access. Caches are normally transparent or hidden from the accessing device.
CAD - Computer Aided Design - Highly graphically oriented software applications which aid in engineering or other types of design.
CD-ROM - Compact Disk Read-Only Memory. A read-only storage device which retrieves up to 640 Mbytes of information from a removable laser disk similar to an audio compact disk.
Command Overlapping - A feature which allows the simultaneous execution of more than one I/O command by a peripheral controller.
Command Packet - An area of host computer memory which is used to store an I/O command and associated parameters for a peripheral controller which uses EATA protocol. Commands for DPT adapters are stored in Command Packets.
Command Queuing - A feature which allows multiple I/O commands to be executed by a peripheral controller, in a more efficient order.
Degraded Mode - The mode of operation of a Redundant Array in which it can continue to be accessed after a component drive has failed. For read accesses, data is synthesized from the remaining good drives in the array. For write accesses, data is stored on the remaining good drives in such a way that is can be restored to the failed drive once it is replaced.
Device - According to the SCSI specification, up to fifteen SCSI devices may be connected to a single SCSI bus. SCSI devices include peripherals such as disk and tape drives, optical devices, scanners, printers, and host adapters. Each SCSI device is assigned a SCSI ID number from 0-7 for 8-bit SCSI, or 0-15 for Wide SCSI.
Differential - An electrical signal protocol which transmits information through a current loop rather than by changes in voltage, thereby reducing the susceptibility to electrical interference. Differential SCSI uses RS-485 transceivers to transfer 10MHz data at distances up to 25 meters (82 feet).
Dirty - A cache page in which data has been written or modified but which has not yet been copied to the storage device. Once the data has been copied to disk, the page is said to be clean.
DMA - Direct Memory Access - A method of data transfer which allows data to be moved between a peripheral controller and system memory without interaction with the host CPU. The data may be moved by the peripheral controller itself, or by a separate 3rd-party DMA controller.
DPT OS Setup Diskette - A diskette created by the Storage Manager utility containing Storage Manager software modules and driver updates for the operating system which is to be installed.
Driver - A software routine which receives I/O requests from higher levels within the operating system and converts those requests to the protocol required by a specific hardware device such as a SCSI adapter.
Duty Cycle - The percentage of time in which an operation is active.
EATA - Extended AT Attachment - The hardware-level high performance protocol used by all DPT products to interface to the host computer. EATA supports Bus Mastering, Scatter/Gather data transfers, and overlapped and queued commands. Contact DPT for a copy of the EATA specification.
ECC - Error Correcting Code - A method of generating redundant information which can be used to detect and correct errors in stored or transmitted data. ECC is stored on disk drives at the end of every sector to correct errors caused by media flaws. ECC is used in SmartRAID adapters to detect and correct errors caused by cache RAM defects or electrical interference over the SCSI bus.
ECU - EISA Configuration Utility - A software utility program provided by the manufacturer of an EISA computer. It is used to configure EISA option cards instead of manually setting jumpers.
EISA - Extended Industry Standard Architecture - An enhanced version of the PC/AT architecture which provides backward compatibility with existing 8 and 16-bit hardware cards. In addition, EISA supports 32-bit data paths, 33 Mbyte/sec data transfers from Bus Mastering peripheral cards, automatic configuration, and a more sophisticated I/O addressing scheme.
Elevator Sorting - A method of sorting records or cache pages by physical location on disk so that the information may be written to disk with less seek and rotational latency.
Failed - The mode of operation of a drive or array in which the drive or array, because of a malfunction, can no longer be accessed.
Fast SCSI - The original SCSI specification defined synchronous data transmission rates of up to 5MHz. By assuming transceivers which provide tighter timing margins, the SCSI-2 standard allows synchronous transfers of up to 10MHz. This provides a transfer speed of 10MB/s for an 8-bit bus, and 20MB/s for a 16-bit bus. Devices which utilize these faster timings are called Fast SCSI devices.
Fast-20 SCSI - The SCSI-3 specification defined synchronous data transmission rates of up to 20MHz. The 20MHz transfer rate provides speeds up to 20MB/s on an 8-bit bus and 40MB/s on a 16-bit bus. Devices that support these timings are called Fast-20 SCSI devices.
Flash ROM - A ROM on the adapter containing firmware that can be reprogrammed via software without removing it from the board.
Flush - Copying all dirty data in the cache to disk.
Function Button - An object used in a graphical user interface, on which the user can point and click with a mouse, to execute a specific function.
GUI - Graphical User Interface. A software interface which interacts with the user through color graphics and a mouse.
Hardware Array - A group of disk drives which are all members of the same RAID-1 or 5 array implemented in host adapter hardware. Multiple Hardware Arrays can be combined into a common Array Group.
HBA - Host Bus Adapter - Refers to the SmartRAID, SmartCache or other peripheral controller cards which provide host computer access to the SCSI bus.
Hit - A data access in which the requested data is found in the cache.
Hit Ratio - The ratio of cache hits to total disk accesses. A hit ratio of 100% means that all disk accesses can be serviced from the cache.
Hot Plug - The operation of adding or removing a device from a bus while transactions involving other devices are occurring over the bus.
Hot Spare - A spare disk drive which, upon failure of a member of a redundant disk array, will automatically be used to replace the failed disk drive.
Hot Swap - The operation of removing a failed disk drive which is a member of a Redundant Array and replacing it with a good drive, while transactions involving other devices are occurring over the bus.
ID - A number from 0 to 7 (or 0 to 15 for 16-bit Wide SCSI) which SCSI devices use to address one another. SCSI IDs are typically selected by setting jumpers on the SCSI device.
Icon - A graphical representation of an object or function used by a GUI.
ISA - Industry Standard Architecture - the name given to the original IBM PC/AT 16-bit bus architecture.
Latency - The time required by a device to access stored data, excluding the data transfer time. Reducing disk latency results in more I/O operations per second being performed on a disk drive.
LRU - Least Recently Used - A cache management algorithm employed by SmartRAID to determine the next cache page to delete and re-use once all pages have been filled with data. By deleting the page in cache which has gone the longest without an access, the LRU algorithm typically results in the most frequently accessed data being cache resident.
LSU - Logical Storage Unit - A logical device on which the computer can store and retrieve information. An LSU may represent an individual disk drive or an Array Group.
LUN - Logical Unit Number - Each SCSI device may contain up to eight sub-devices or Logical Units, assigned Logical Unit Numbers 0-7. Typically, SCSI devices such as a disk or tape drives contain only LUN 0.
Menu - An item at the top of a GUI display which, when selected by clicking with a mouse, will cause a list of related functions to be displayed.
Mirroring - A popular term for RAID-1. A method of creating disk-fault tolerance by redundantly storing information on pairs of drives.
Miss - A data access in which the requested data is not found in cache. Cache misses result in the normal average disk latency of 20ms when accessing data.
MTBF - Mean Time Between Failure. The average time between expected failures of a device.
Non-Redundant Array - An Array Group with no fault tolerance (RAID-0). If one drive in a Non-Redundant Array fails, the entire Array Group will fail.
Optimal - The mode of operation of a disk array in which no drive failures have occurred.
OS - Operating System - Software which manages the computer's resources and provides the operating environment for application programs.
Page - The smallest region of cache which may be allocated to storing data. The SmartRAID adapter can contain up to 128,000 pages, each storing one 512-byte sector from disk.
Parity - A method of generating redundant information which can be used to detect errors in stored or transmitted data. Parity is used in standard RAM SIMMs and over the SCSI bus to detect data errors. Parity is used in RAID-5 disk arrays to reconstruct flawed or missing data sectors.
PCI - Peripheral Component Interconnect - An intelligent computer bus specification which supports 32-bit data paths, 132 Mbyte/sec data transfers from Bus Mastering devices and plug-and-play automatic configuration of peripheral cards.
PIO - Programmed Input/Output - The method of data transfer used whereby data is moved between the peripheral controller and system memory via the CPU. PIO transfers typically occur at about 2.5MB/s.
Plug-and-Play - The ability to install peripheral cards or devices without requiring manual configuration by the user.
Plug-and-Play BIOS - The system BIOS in PCI computers which has the ability to automatically configure PCI peripheral cards so as to avoid contention with other installed devices.
RAID - Redundant Array of Inexpensive Disks - A method of combining hard disks into one logical storage unit which offers disk-fault tolerance and can operate at higher throughput levels than a single hard disk.
Rebuild - The operation of recreating data belonging to a failed member of a redundant disk array and writing that data onto a replacement disk drive.
Redundant Array - A fault tolerant Array Group. (RAID-1 or RAID-5).
Scatter/Gather - A feature which allows data to be transferred to or from multiple discontiguous areas of host computer memory with a single I/O command.
SCSI - Small Computer Systems Interface - an ANSI standard parallel interface designed to communicate with intelligent peripheral devices. The SCSI definition, created by ANSI is defined in document number X3.131-1986. Copies of this document can be obtained from the ANSI X3 Secretariat.
SIMM - Single In-line Memory Module - A standard way of packaging RAM on a small circuit board with a defined edge connector. SmartRAID adapters can use any combination of 1MB, 4MB or 16MB 36-bit single-sided low-profile RAM SIMMs. DPT also offers 4MB and 16MB ECC SIMMs. When used with SmartRAID Storage Cabinets, DPT ECC SIMMs are required for ECC protection over the SCSI bus.
Single-Ended - An electrical signal protocol which transmits information through changes in voltage. Single-Ended SCSI uses standard TTL signal-and-ground pairs to transmit information over the SCSI bus.
Status Packet - An area of host computer memory which is used to store the completion status for an I/O command for a peripheral controller which uses EATA protocol. SmartRAID adapters return command completion status in Status Packets.
Stripe - A contiguous region of disk space. Stripes may be as small as one sector or may be composed of many contiguous sectors.
Striping - Also called RAID-0. A method of distributing data evenly across all drives in an array by concatenating interleaved stripes from each drive.
Synchronous - Data transmission protocol which is synchronized to a defined time interval. Synchronous SCSI can transmit data faster than asynchronous SCSI because the transmitting device does not wait for acknowledgement of each byte from the receiving device. Instead, it continues to transmit data at the rate negotiated by both devices (up to 20MHz).
Tagged Command Queuing - A feature of SCSI-2 and SCSI-3 protocols that allows SCSI commands to be executed out of order.
Termination - A method of matching the transmission impedance of a electrical bus so as to eliminate signal reflections from the physical ends of the bus.
TERMPWR - A signal on the SCSI bus which provides power for SCSI bus terminators on remote devices.
Throughput - A term used to describe the amount of data which can be processed by a system in a given amount of time.
Tool Bar - A bar, near the top of the screen used by a GUI, which contains Function Buttons.
Ultra SCSI - Another name used for Fast-20 SCSI.
Write-Back - A method postponing data to be written to a slow device such as a disk drive by temporarily saving the data in a cache. The data can then be written at a time when the device would otherwise be idle. In addition, the data can be processed through techniques such as elevator sorting, so that the write operations will go faster.
Write-Through - The opposite of write-back. In write-through mode, data must be written to the final destination before a write operation is completed.
Wide SCSI - A SCSI protocol and signal definition which provides a greater than 8-bit wide data path. Wide SCSI devices may support either 8 and 16-bit, or 8, 16 and 32-bit data transfers.
WORM - Write Once Read Multiple - An optical storage device similar to a CD-ROM which can write data one time only to any location on a removable laser disk. WORMs are primarily used as data archive devices because once written, the data can never be erased.
XOR - Exclusive OR - A logical operation performed on two binary operands which yields a 0 in every bit position where the operands are both 1 or both 0, and a 1 in every bit position when the operands are dissimilar. Performing XOR operations on every bit in a word generates a parity bit for that word.