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Cache
Pronounced “cash”. An external or internal reserved portion of a computer’s electronic memory Random-Access Memory (RAM). Frequently used information is stored in the cache so that your computer can get the information more quickly.

CD
The 12cm (4.75 in.) optical read only disc used for digital audio, data, and video storage and retrieval in various computer, audio, and video systems. CDs are produced using a number of processes to store information for optical retrieval. see: WORM, WREN, CD-ROM

CD- DA Track
a track on a compact disc containing audio information encoded according to the CD-Digital Audio specific

CD-I
Compact Disc Interactive. An interactive audio/video/computer system developed by Sony and Phillips for the consumer market.

CD-ROM
Compact Disc Read Only Memory. An adaptation of CD technology for use with general digital data. CD-ROM discs are “pressed” in an injection molding process from a master mould to create the data tract. Then they are coated with a reflective material and sealed.

CD-ROM XA
Compact Disc Read Only Memory Extended Architecture. A format for recording compressed digital audio at lower qualities, allowing capacity increases on a single CD from four to 19 hours.

CD-RTOS
Kernel: the nucleus of CD-RTOS, which is responsible for service request processing, memory management, system initialization, multi-tasking, input/output management and exception and interrupt processing.

CD-RTOS
Compact Disc Real-Time Operating System. the name of the operating system used in CD-I players.

CD-WORM
Compact Disc Write Once Read Many times. A type of CD-ROM disc named for the process used to create data on the disc. CD-WORM Discs perform identically as CD-ROM discs. The difference being a laser is used to “burn” the reflective layer to create the data tract. The cost of ” WORM Burning” is considerably less than glass master production for a single disc.CD-WORM Disks can be used to transport CD-ROM s for mastering and replication.

CD-WREM
Compact Disc Write Read Erase Memory. A data storage system commonly referred to as “Magneto-Optical”. It incorporates laser technology to “Burn” a magnetic layer on the disc, this produces a reflective surface that can then be read. This magnetic layer can be burned many times giving the medium read, write, and write over capability not found in CD- ROM & CD-WREM. This advantage comes only with considerable cost for Drives and Disc Cartridges. As a result this technology has not caught on in the home consumer market. It has great potential for multimedia, pre-press publication, and other digital graphic applications.

chroma keying
facility to replace selected colors in a video image with others that allows the creation of different scenes against the background. Some video boards contain such capabilities.

Chrominance
signals of an image system that represent the color components of the image such as hue and saturation. A black and white image has chrominance value of zero.

Clipart
Stock digital images in various subject matter and format for use in multimedia productions or anywhere graphics are needed. Rights to use, pricing, and quality vary from one manufacturer to another.

Clone
To duplicate a portion of an image to another location. This is a cornerstone to the editing of digital images. This effect is useful in the process of retouching photographs.

CLUT
Color Look-Up Table. A table containing all the colors that may be used in a particular picture. Each entry provides an RGB value. The picture may then be encoded using the table entry addresses rather than the direct RGB value.

Color balancing
The process of tuning the overall color cast of an image or selection

Compressiona
digital process that permits data to be stored or transmitted using less than normal the number of bits. Compression is critical for displaying audio and moving video fast enough on desktop computers. Some compression standards for still images are formulated by JPEG, for moving video by MPEG, and for telecommunications transmission by CCITT.

Data Storage

CD Optical Storage Glossary of Computer Terms (Letter B)

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Bitmap
name for a family of image file-types composed of dots (pixels). Bitmaps have attributes specifying color format, resolution, header, bit order, and other variables. A checker board is a good conceptual model for a bitmap image.

Blur
To distort the current selection or image by reducing contrast along object lines with gradients.

BPP
Bits Per Pixel. An acronym for the number of bits used to represent the color value of each pixel in a digital image color format. Example: CMYK=32bpp, RGB true color =24bpp,256 color & grayscale=8bpp, 16color=4bpp, Black and White line art=1bpp.

Brightness & contrast balancing
The process of adjusting the density and intensity of an image.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter S)

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SCSI
Small computer system interface (pronounced scuzzy). The fast, intelligent input/output parallel bus used by high-performance peripherals.

Software-based array
An array in which all management functions including parity calculation (XOR) are performed by the host server CPU. These products are low priced but have high CPU utilization and limited fault-tolerant features. High-performance, low-cost array adapters are quickly replacing these inferior software-based arrays.

System disk
The disk (or array) on which a system’s operating system is stored and from which it is initially loaded into system memory.

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. Many other leading server, storage, and RAID controller manufacturers worldwide have endorsed the SAF-TE specification. 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. All Mylex RAID controllers feature SAF-TE.

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

Segment Size
See Cache Line Size

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.

SES
SCSI Enclosure Services, a standard for SCSI access to services within an enclosure containing one or more SCSI devices. For disk drives, power supplies, cooling elements, and temperature sensors, the actions performed are the same as for SAF-TE monitoring. If a UPS is connected to any SES-monitored enclosures, and an AC failure or two minute warning is reported, conservative cache is enabled and all system drives are switched to write-through cache. Primarily used in fibre enclosures.

Session
The period of time between any two consecutive system shutdowns; system shutdown may be either a power off/on, or a hardware reset.

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

Spanning
A process that provides the ability to configure multiple drive packs or parts of multiple drive packs. In effect, spanning allows the volume used for data processing to be larger than a single drive. Spanning increases I/O speeds, however, the probability of drive failure increases as more drives are added to a drive pack. Spanned drive packs use striping for data processing. See also Striping and Drive Groups, Drive Packs.

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

Standby Replacement of Disks
See also 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 controller automatically carries out the rebuild operation 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, and RAID 0+1.

Note: The standby rebuild will only happen on the same DAC960 controller, never across DAC960 controllers.

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 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 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
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. 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.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter R)

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RAID
Redundant Array of Independent Disks, a collection of two or more disks working together in an array. Mylex RAID controllers implement this technology to connect up to 15 SCSI devices per channel. The different forms of RAID implementation are known as “RAID levels.” See also Berkeley RAID Levels, Disk Array, and RAID Levels.

The system manager or integrator selects the appropriate RAID level for a system. 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
See RAID Controller

RAID Advisory Board (RAB)
An association of companies whose primary intention is to standardize RAID storage systems. Mylex is a member of RAB.

RAID Controller
Low cost RAID controllers that use SCSI channels on the motherboard.

RAID Levels
Mylex disk array controllers support four RAID Advisory Board approved (RAID 0, RAID 1, RAID 3, and RAID 5), two special (RAID 0+1, and JBOD), and three spanned (RAID 10, 30, and 50) RAID levels. All DAC960, AcceleRAID, and eXtremeRAID series controllers support these RAID levels. See also Berkeley RAID Levels.

– Level 0:
Provides block “striping” across multiple drives, 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 and better performance than Level 1 alone.

– 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 feature in RAID subsystems that allows for changing a RAID level to another level without powering down the system.

Read-Ahead Cache
A caching strategy whereby the computer anticipates data and holds it in cache until requested.

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 the system requires to carry out its functions).

Rotated XOR Redundancy
XOR refers to the Boolean “Exclusive-OR” operator. Also known as Parity, 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 amongst 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. In any RAID 3 or RAID 5 array, the capacity allocated to redundancy is the equivalent of one drive.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter P)

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Parity
A method of providing complete data redundancy while requiring only a fraction of the storage capacity of mirroring. The data and parity blocks are divided between the disk drives in such a way that if any single disk drive is removed or fails, the data on it can be reconstructed using the data on the remaining disk drives. The parity data may exist on only one disk drive or be distributed between all disk drives in a RAID group. See also Rotated XOR Redundancy.

Parity Check
A function used to verify the integrity of data on a system drive. It verifies that mirror or parity information matches the stored data on the redundant arrays. If the parity block information is inconsistent with the data blocks, the controller corrects the inconsistencies. See also Consistency Check.

Partitioning
Where the full usable storage capacity of a disk or array of disks appears to an operating environment in the form of several virtual disks whose entire capacity approximates that of the underlying disk or array.

PCI Hot Plug
A feature that allows for the printed circuit board (PCB) to be replaced without powering down the entire system-an essential feature in newer PCI-based PCs. Mylex DAC960PG, DAC960PJ, AcceleRAID, and eXtremeRAID products are all PCI Hot Plug compatible. See also Hot Plug.

Parity
A form of data redundancy used by RAID levels 2, 3, 4, and 5 to recreate the data of a failed drive in a disk array.

Pre-fetching
Intelligent gathering of data from disks prior to requests from the operating system.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter M)

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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.

Mirrored Hard Drive
Two hard drives the computer sees as one unit. Information is stored simultaneously on each drive. If one hard disk drive fails, the other contains all of the cached data and the system can continue operating.

Mirroring
The complete duplication of data on one disk drive to another disk drive, this duplication occurs simultaneously with each write operation: each disk will be the mirror image of the other (also known as RAID Level 1, see RAID Levels). All Mylex RAID controllers support mirroring.

MTBF
Mean time between failure. Used to measure computer component average reliability/life expectancy. MTBF is not as well-suited for measuring the reliability of array storage systems as MTDL, MTTR or MTDA (see below) because it does not account for an array’s ability to recover from a drive failure. In addition, enhanced enclosure environments used with arrays to increase uptime can further limit the applicability of MTBF ratings for array solutions.

MTDA
Mean time between data access (or availability). The average time before non-redundant components fail, causing data inaccessibility without loss or corruption.

MTDL
Mean time to data loss. The average time before the failure of an array component causes data to be lost or corrupted.

MTTR
Mean time to repair. The average time required to bring an array storage subsystem back to full fault tolerance.

Member (disk)
A disk that is in use as a member of a disk array.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter L)

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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
The logical devices presented to the operating system. System drives are presented as available disk drives, each with a capacity specified by the Mylex RAID controller. See also Storage Device.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter J)

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JBOD
Just A Bunch of Disks (Drives), a number of disk drives, usually in an enclosure. JBOD implies that the disks do not use RAID technology and function independently. All Mylex RAID controllers support JBOD mode.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter I)

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Immediate RAID Availability
See Background Initialization

In-Line Terminator
A plug attached to the end of a SCSI cable in order to initiate active termination. Used when SCSI devices on the cable do not have built-in termination. See also Active Termination.

Interface
A hardware or software protocol that manages the exchange of data between the hard disk drive and the computer. The most common interfaces for small computer systems are ATA (also known as IDE) and SCSI.

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.
Examples of internal RAID controllers include the Mylex AcceleRAID and eXtremeRAID families.

Data Storage

RAID Array & Server Glossary of Computer Terms (Letter H)

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

Host adapter
A bus-based (PCI, EISA, ISA) hardware device, such as an add-in card or ASIC, that converts the timing and protocol of a host’s memory bus and an I/O bus.

Hot spare
RAID storage feature that allows a spare drive (or other component) to be configured for automatic (in contrast to hot-swap) replacement and reconstruction in the event of a disk failure. Users can remain on-line and continue to access data.

Hot Replacement of Disks
The design of all Mylex 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), the system generates a message 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. See also Hot Swap.Hot SpareA physical disk drive not part of a system drive that the controller can use to automatically rebuild a critical system drive. The hot spare drive must have at least as much capacity as the largest disk drive in the array or the rebuild may not start. See also Hot Standby and Standby Replacement of Disks.

Hot Standby
A redundant component in 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. See also Hot Replacement of Disks and Hot Spare.