Array Two or more disks grouped together to appear as a single disk to the host system.
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.
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.
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).
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.
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).
ECC (Error Correction Code) Refers to parity error detection and correction within memory or cache (for example, SIMM). Depending on the ECC SIMM, single-bit or double-bit parity errors can be detected but not corrected, or detected and corrected automatically.
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.
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.
Courtesy of:
Adaptec : http://www.adaptec.com/products/guide/arrayguideglossary.html
Adaptec : http://www.adaptec.com/technology/whitepapers/raid_hw_sw07.html
Maximum PC : http://www.maximumpc.com/terminator/index.html
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