Disk array controller: Difference between revisions

Revision as of 09:13, 27 October 2011

Revision as of 09:13, 27 October 2011

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

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

== A single controller ''may'' use different protocols for back-end and for front-end communication. Many enterprise controllers use FC on front-end and SATA on back-end.

Disk array controller provides front-end interfaces and back-end interfaces.

* Back-end interface communicates with controlled disks. Hence protocol is usually [[Advanced Technology Attachment|ATA]] (a.k.a. PATA; incorrectly called IDE), [[Serial ATA|SATA]], [[SCSI]], [[Fibre Channel|FC]] or [[Serial Attached SCSI|SAS]].

* Front-end interface communicates with a computer's [[host adapter]] (HBA, Host Bus Adapter) and uses:

** one of ATA, SATA, SCSI, FC; these are popular protocols used by disks, so by using one of them a controller may transparently [[emulate]] a disk for a computer

** somewhat less popular protocol dedicated for a specific solution: [[FICON]]/[[ESCON]], [[iSCSI]], [[HyperSCSI]], [[ATA over Ethernet]] or [[InfiniBand]]

== A single controller ''may'' use different protocols for back-end and for front-end communication. Many enterprise controllers use FC on front-end and SATA on back-end.

== Enterprise controllers ==

{{main|Disk array}}

In a modern enterprise architecture disk array controllers are parts of physically independent [[disk enclosure|enclosure]]s, such as [[disk array]]s placed in a [[storage area network]] (SAN) or [[network-attached storage]] (NAS) [[Server (computing)|server]]s.

Those external disk arrays are usually purchased as an integrated subsystem of RAID controllers, disk drives, power supplies, and management software. It is up to controllers to provide advanced functionality (various vendors name these differently):

* automatic [[failover]] to another controller (transparent to computers transmitting data)

* long-running operations performed without [[downtime]]

** forming a new RAID set

** reconstructing ''degraded'' RAID set (after a disk failure)

** adding a disk to online RAID set

** removing a disk from a RAID set (rare functionality)

** partitioning a RAID set to separate volumes/LUNs

* [[snapshot (computer storage)|snapshots]]

* [[Business Continuance Volume]]s (BCV)

* [[disk mirroring|replication]] with a remote controller....

== Simple controllers ==

[[Image:Raid controller.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]] [[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 (3Gbit/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 were available for a long time, they always required expensive [[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 access|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 the users wanted the fault-tolerance of RAID without investing in expensive SCSI drives.

ATA 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.

== References ==