Disk array controller: Difference between revisions

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{{Short description|Device that manages disk drives}}

A '''disk array controller''' is a device that manages the physical [[disk drives]] and presents them to the computer as [[Logical Unit Number|logical units]]. It almost always implements [[RAID#Hardware-based|hardware]] [[RAID]], thus it is sometimes referred to as '''RAID controller'''. It also often provides additional disk [[cache (computing)|cache]].

A '''disk array controller''' is a device that manages the physical [[disk drives]] and presents them to the computer as [[Logical Unit Number|logical units]]. It almost alwaysoften implements [[RAID#Hardware-based|hardware]] [[RAID]], thus it is sometimes referred to as '''RAID controller'''. It also often provides additional disk [[cache (computing)|cache]].

''Disk array controller'' is often improperly shortened to ''[[disk controller]]''. The two should not be confused as they provide very different functionality.

''Disk array controller'' is often improperlyambiguously shortened to ''[[disk controller]]''. Thewhich twocan shouldalso notrefer beto confusedthe ascircuitry theyresponsible providefor verymanaging internal disk differentdrive functionalityoperations.

== Front-end and back-end side ==

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[[Image:Promise Ultra33.jpg|thumb|250px|Promise Technology ATA RAID controller]]

A simple disk array controller may fit inside a computer, either as a [[Peripheral Component Interconnect|PCI]]/[[PCIe]] [[expansion card]] or just built onto a [[motherboard]]. Such a controller usually provides [[Host adapter|host bus adapter]] (HBA) functionality itself to save physical space. Hence it is sometimes called a '''RAID adapter'''.

{{As of | 2007 | February }} [[Intel]] started integrating their own [[Intel Matrix RAID|Matrix RAID controller]] in their more upmarket motherboards, giving control over 4 devices and an additional 2 SATA connectors, and totalling 6 SATA connections (3Gbit3 Gbit/s each). For backward compatibility one IDE connector able to connect 2 ATA devices (100  Mbit/s) is also present.

=== History ===

While hardware RAID controllers werehave been available for a long time, they alwaysinitially required expensive [[Parallel SCSI]] hard drives and aimed at the server and high-end computing market. SCSI technology advantages include allowing up to 15 devices on one bus, independent data transfers, [[hot-swapping]], much higher [[MTBF]].

Around 1997, with the introduction of [[Atapi|ATAPI-4]] (and thus the [[Direct memory accessUDMA|Ultra-DMA-Mode 0]], which enabled fast data- transfers with less [[CPU]] utilization) the first ATA RAID controllers were introduced as PCI expansion cards. Those RAID systems made their way to the consumer market, where thefor users wantedwanting the fault-tolerance of RAID without investing in expensive SCSI drives.

ATAFast consumer drives make it possible to build RAID systems at lower cost than with SCSI, but most ATA RAID controllers lack a dedicated buffer or high-performance XOR hardware for parity calculation. As a result, ATA RAID performs relatively poorly compared to most SCSI RAID controllers. Additionally, data safety suffers if there is no [[Battery (electricity)|battery]] backup to finish writes interrupted by a power outage.

==OS support==