RAID Array & Server Glossary of Computer Terms (Letter G)
Global Spare
An extra, physical disk drive placed in an array and used as a Hot Spare. A global spare automatically takes the place of a failed drive. See also Auto Swap.
General Computer Terminology
RAID Array & Server Glossary of Computer Terms (Letter F)
Failback
Restoring 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 a redundant component has assumed its functions.
Failover Port
A fibre channel port capable of assuming I/O requests for another, failed port on the loop. During normal operation, a failover port may be active or inactive. Failover ports assume the same loop ID and, optionally, the same node from the failed port.
Failure
A detectable physical change in hardware, requiring replacement of the component.
Fault Tolerance, Failure Tolerance
The ability of a system to continue to perform its function even when one of its components has failed. A fault tolerant system requires redundancy in disk drives, power supplies, adapters, controllers, and cabling. Mylex RAID controllers offer high levels of fault tolerance.
Failed-drive mode
A mode of reduced-performance operation that a disk array is in after a drive failure.
Failover
The automatic replacement of a failed system component with a properly functioning one. Most often used in the context of redundant external array controllers. If one of the controllers fails, failover enables the second controller to take over the failed controller’s I/O load.
Fault tolerance
The ability of a system to continue to perform its functions, even when one or more components have failed
Fibre Channel
Technology for transmitting data between computer devices at a data rate of up to 2 Gbps (two 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 channels also work using coaxial cable and ordinary telephone twisted pair wires.
Flash ROM
Memory on an adapter containing software that can be reprogrammed without removing it from the board.
RAID Array & Server Glossary of Computer Terms (Letter E)
ECC
Error Correcting Code, a method of generating redundant information which can be used to detect and correct errors in stored or transmitted data.
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.
External RAID Controller
A RAID controller in its own enclosure, rather than incorporated into a PC or server. External RAID controllers are often referred to as a Bridge RAID Controller. Mylex SANArray FL, FF, FFx, and Pro FF2 controllers are external RAID controllers. Compare with Internal RAID Controller.
Exclusive OR (XOR)
A process based on a mathematical algorithm that is used by RAID levels 2, 3, 4, and 5 to compare computer data (binary 0s and 1s) created by a write request or by a read request during a drive failure. The result of the XOR process is parity information that will be stored along with data for real-time recovery in the event of a disk failure.
External array controller
In contrast to bus-based array adapters and microprocessor-based array controllers, external array controllers reside in the external RAID storage enclosure. They connect to the host through a standard SCSI or serial (such as Fibre Channel) host adapter interface. These external controllers are similar to bus-based, microprocessor-based array controllers, in that they include an on-board microprocessor to offload all RAID functions (I/O commands and RAID operations) from the host CPU. They are usually used in midrange and high-end servers, especially in clustering environments.
RAID Array & Server Glossary of Computer Terms (Letter D)
Data Caching
Temporary storage of new write data or high-demand read data in solid state memory in order to accelerate performance. The cached data is later overwritten with newly cached data once it is either written to disk or deemed to be of low demand.
Demand caching
A performance caching technique in which the currently requested data is read in anticipation of another request before its allocated blocks are recycled. Reassignment of the blocks is done on the basis of least recently used (LRU).
Degraded Mode
A RAID mode used when a component drive has failed.
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, allow disk drives to be used together to improve fault tolerance, performance, or both. Disk arrays are commonly used on servers and are becoming more popular on desktops and workstations. See also Array.
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 controller could take the disk “offline.” Mylex DAC960 controllers also monitor SCSI bus parity errors and other potential problems. Any disk with too many errors will also be taken “offline.” See also Offline.
Disk Traveling, Drive Traveling
A process that occurs when the drives are placed in a different order than the original order. Disk traveling can occur whether or not a drive has failed.
Disk System
A storage system capable of supporting only disks.
Drive Groups, 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. 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 (5×200), not 1800MB.
Disk/data striping
Spreading data evenly over multiple disks to enhance performance. Sometimes referred to as RAID 0, data striping actually has no redundancy scheme and, therefore, does not provide any fault tolerance (data protection).
Drive
Synonym for disk, hard drive, hard disk, disk drive.
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.
Duplexing
Mirroring across two host adapters. Used only with software-based RAID storage systems (usually the embedded network operating system RAID software such as NetWare and Windows NT).
RAID Array & Server Glossary of Computer Terms (Letter C)
Cache
A temporary storage area for frequently accessed or recently accessed data. Cache is used to speed up data transfer to and from a disk. See also Caching.
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
Represents the size of the data “chunk” that will be read or written at one time, and is set in conjunction with stripe size. Under RAID EzAssistTM, the cache line size (also known as Segment Size) should be based on the stripe size you selected. The default segment size for Mylex RAID controllers is 8K. See also Stripe Size.
Caching
Allows data to be stored in a pre-designated area of a disk or RAM. Caching speeds up the operation of RAID systems, disk drives, computers and servers, or other peripheral devices. See also Cache.
Conservative Cache
An operating mode in which system drives configured with the Write-Back Caching policy are treated as though they were configured for Write-Through operation and the cache is flushed.
Consistency Check
A process that verifies the integrity of redundant data. A consistency check on a RAID 1 or RAID 0+1 configuration (mirroring) checks if the data on drives and their mirrored pair are exactly the same. For RAID Level 3 or RAID Level 5, a consistency check calculates the parity from the data written on the disk and compares it to the written parity. A consistency check from Mylex utilities such as Global Array ManagerTM (GAM) or RAID EzAssistTM give the user the ability to have a discrepancy reported and corrected. See also Parity Check.
Controller
An adapter card, RAID controller, or other module that interprets and controls signals between a host and a peripheral device.
Data Storage Glossary
A
1394
The Institute of Electrical and Electronics Engineers (IEEE) designation for an interface developed for easy connection to consumer devices such as video and computer peripherals.
Adapter Card
In order to connect a computer to peripheral devices, an adapter card is often required. The adapter plugs into the computer’s bus, and connects the system’s data path to the peripheral.
Arrays
A group of disk drives that appear to a computer as a single logical unit. In order to use arrays effectively, RAID software and/or hardware is required.
B
Bandwidth
A data transmission rate; the maximum amount of information (bits/second) that can be transmitted along a channel.
Bus
A set of conductors connecting the various functional units in a system.
C
CD-R (Compact Disc-Recordable)
A blank CD that is designed to allow data to be “written” a single time on its surface. Peripheral devices that connect to PCs enable home or office single-time recording of blank CD-Rs.
CD-RW (Compact Disc-Rewritable)
A blank CD that is designed to allow data to be written, erased, and rewritten onto a CD-RW. Rewritable capability makes CDs more versatile by mimicking the usability of floppy disks.
D
Data Caching
Temporary storage of new write data or high-demand read data in solid-state memory in order to accelerate performance. The cached data is later overwritten with newly cached data once it is either written to disk or deemed to be of low demand.
Data Path
Digital information can be transmitted in different sized “paths” within a computer. Generally, the wider the data path, the higher the throughput performance. Today, the most standard data path width is 32-bit, though new products are entering the market with 64-bit paths, providing better I/O performance.
Disk Drive
Computer storage hardware that can read and write information on it.
E
EID
Enhanced Integrated Drive Electronics. A low cost, limited functionality drive interface. Controlled by the ANSI X3T9.2 committee.
Ethernet
A type of networking technology for local area networks.
F
Fault Tolerance
The ability of a system to continue to perform its functions, even when one or more components have failed.
Fibre Channel
A channel/network standard that provides connectivity, distance, and protocol multiplexing.
I
I/O Bus
The I/O bus is where the computer connects to outside peripherals.
I/O Channel
In computer systems, I/O Channel refers to the physical interface that controls the transfer of data between the computer and peripheral devices. With SCSI, each I/O Channel is equivalent to the full functionality of a single SCSI host adapter. For example, a dual-channel SCSI host adapter is equivalent to two single SCSI host adapters.
I/O Subsystem
The combination of technologies that manage the process of moving data into and out of the main computer system. The highest performance I/O subsystems use dedicated processors to minimize the CPU’s need to manage I/O, thereby allowing it to process the information that is moved to it from the I/O subsystem.
M
Mirroring
Also known as RAID 1 or duplexing (when using two host bus adapters). Full redundancy is obtained by duplicating all data from a primary disk on a secondary disk. The overhead of requiring 100% data duplication can get costly when using more than two drives.
Motherboard
The main printed circuit board in a system generally containing the bus, microprocessor, and chips used for controlling any internal peripherals.
Multitasking
The ability for the operating system to perform multiple operations at once. Windows NT Workstation is a multitasking operating system that can perform multiple I/O requests at once. SCSI and a Caching RAID coprocessor take advantage of multitasking.
N
Network Interface Card (NIC)
An adapter installed in a computer to provide a physical connection to a network.
P
Parity
When the data stream is split between several disks with an extra disk providing error protection.
PCI (Personal Computer Inter-connect)
PCI is the most common high-performance bus type. Currently, PCI uses a 32-bit wide data path, but newer PCI products are adopting a 64-bit wide data path for improved performance.
Peripheral
Internal or external devices connected by cable to a system.
R
RAID (Redundant Array of Independent/Inexpensive Disks)
A method of combining multiple disk drives into a single logical storage unit. Multiple levels of RAID provide different features. RAID Level 0 is the fastest type of RAID. It stores data across all the drives, letting users access information from multiple drives simultaneously. RAID Level 1 protects data by mirroring it on multiple drives, so performance is only slightly better than that of a single drive. RAID Level 5 does a combination of the two, providing the best overall balance.
Read-ahead Caching
A performance caching technique in which data is anticipated and read into the cache before it is actually requested.
S
SAN (Storage Area Network)
SANs are an evolving approach to storage, where multiple storage devices are connected to multiple servers for higher capacity, throughput, and reliability. SANs require sophisticated RAID management software and high-performance I/O connectivity.
SCSI (Small Computer System Interface, pronounced “scuzzy”)
SCSI is the preferred industry standard for high-performance I/O interface. Particularly valuable in servers where one system must connect to many high-capacity storage devices without lowering the I/O speed to the slowest device.
S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology)
Drives equipped with this feature report predicted failures based on threshold values determined by the manufacturer. This allows the network manager to replace a drive before it fails.
Storage
Computers store information on a variety of devices, some inside the system, and others external to the computer. Typically, data is written to a particular kind of storage medium using a disk drive. Common media include flexible (floppy disk drive), rigid (hard disk drive), tape, or optical (CD).
Striping
Also known as RAID 0. Two or more drives store and retrieve data in parallel, accelerating performance.
W
Write-back Caching
A performance caching technique in which the completion of a write request is signaled as soon as the data is in cache. Actual writing to the disk occurs at a later time.
Write-through Caching
A caching technique in which the completion of a write request is not signaled until data is safely stored on disk
RAID Array & Server Glossary of Computer Terms (Letter B)
Background Initialization
Where the initialization process of a disk array takes place in the background, allowing use of a disk array within seconds instead of several hours. Also known as Immediate RAID Availability.
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/O systems at the University of California at Berkeley. There are six Berkeley RAID levels, usually referred to as RAID Level 0 through RAID Level 5. See also RAID Levels.
Bridge RAID Controller
A device appearing as a single ID in a Storage Area Network (SAN), 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.
Bootable array
An array which includes system disk files and allows a server to boot from the array while protecting the network operating system disk — and other data on the array — from drive failure.
Cold swap Power must be switched off before the removal or insertion of a component.
RAID Array & Server Glossary of Computer Terms (Letter A)
Array
Multiple disk drives configured to behave as a single, independent disk drive. See also Disk Array.
Array adapter
A bus-based (usually PCI) hardware device — such as an add-in card, group of motherboard ASICs, or a combination of both — that converts the timing and protocol of a host’s memory bus and an I/O bus. Usually used in entry-level servers, an array adapter also includes an on-board RAID co-processor to offload most of the RAID operations — for example, secondary RAID 1 writes and RAID 5 parity calculations — from the host CPU. This is in contrast to the microprocessor-based array controllers used in midrange and high-end servers, which also offload I/O commands. Array adapters improve performance over software RAID solutions embedded within network operating systems such as NetWare and Windows NT. These adapters provide the same connectivity functions as a standard host adapter.
ATA
Short for AT Attachment. A hard drive with an integrated controller. There are multiple levels of ATA standards including the base-level 16-bit IDE, ATA-2 (Enhanced IDE), Ultra ATA (ATA33), ATA66 and ATA100. A good explanation and tutorial is available at PC Guide.
ATAPI
ATA Packet Interface. Defines a set of commands supported through the ATA-2 interface for peripherals other than hard drives, such as CD-ROM, DVD-ROM, and tape drives.
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