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 swap A storage system's ability to allow the removal and replacement of a disk drive (or other component) while users are on-line and accessing data. In contrast to hot spare, this is a manual operation. Hot swap requires that the storage (or server) enclosure drive tray connectors be designed so that when a drive is removed, power is disconnected before the ground connection, and that the ground is restored before the power is reconnected upon reinsertion of the drive. This is usually accomplished by making the ground pin(s) in the drive tray connector slightly longer than the data pins.
JBOD Just a bunch of drives. Refers to an array of drives without data redundancy.
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.
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 percent data duplication can get costly when using more than two drives.
PCI Peripheral Component Interconnect Industry-standard specification that refers to a high-speed (133 MB/sec) host bus commonly used for host adapters, Ethernet adapters, and video cards.
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.
RAID Redundant array of inexpensive disks. The term coined in 1987 by researchers at the University of California at Berkeley to describe a series of redundant architectures used in fault-tolerant disk arrays.
RAID levels Numbered 0 through 5, RAID levels refer to different array architectures that offer various advantages in terms of data availability, cost, and performance.
Read-ahead cache A performance caching technique in which data is anticipated and read into the cache before it is actually requested.
Redundant A duplicate disk or component that provides a recovery path in case of a failure.
SCSI Small computer system interface (pronounced scuzzy). The fast, intelligent input/output parallel bus used by high-performance peripherals.
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.
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.
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