Disk array controller
Encyclopedia
A disk array controller is a device which manages the physical disk drives and presents them to the computer as logical units. It almost always implements hardware RAID
, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache
.
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.
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.
s, such as disk array
s placed in a storage area network
(SAN) or network-attached storage
(NAS) 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):
A simple disk array controller may fit inside a computer, either as a PCI
expansion card
or just built onto a motherboard
. Such a controller usually provides host bus adapter
(HBA) functionality itself to save physical space. Hence it is sometimes called a RAID adapter.
Intel started integrating their own 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.
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-4 (and thus the 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
backup to finish writes interrupted by a power outage.
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...
, thus it is sometimes referred to as RAID controller. It also often provides additional disk cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
.
A disk array controller name is often improperly shortened to a disk controller
Disk controller
The disk controller is the circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.Early disk controllers were identified by their storage methods and data encoding. They were typically implemented on a separate controller card...
. The two should not be confused as they provide very different functionality.
Front-end and back-end side
Disk array controller provides front-end interfaces and back-end interfaces.- Back-end interface communicates with controlled disks. Hence protocol is usually ATA (a.k.a. PATA; incorrectly called IDE), SATASerial ATASerial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
, SCSISCSISmall Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
, FCFibre ChannelFibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
or SASSerial Attached SCSISerial Attached SCSI is a computer bus used to move data to and from computer storage devices such as hard drives and tape drives. SAS depends on a point-to-point serial protocol that replaces the parallel SCSI bus technology that first appeared in the mid 1980s in data centers and workstations,...
. - Front-end interface communicates with a computer's host adapterHost adapterIn computer hardware, a host controller, host adapter, or host bus adapter connects a host system to other network and storage devices...
(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: FICONFICONFICON is the IBM proprietary name for the ANSI FC-SB-3 Single-Byte Command Code Sets-3 Mapping Protocol for Fibre Channel protocol. It is a FC layer 4 protocol used to map both IBM’s antecedent channel-to-control-unit cabling infrastructure and protocol onto standard FC services and infrastructure...
/ESCONESCONESCON is a data connection created by IBM, and is commonly used to connect their mainframe computers to peripheral devices such as disk storage and tape drives. ESCON is an optical fiber, half-duplex, serial interface. It originally operated at a rate of 10 Mbyte/s, which was later increased to...
, iSCSIISCSIIn computing, iSCSI , is an abbreviation of Internet Small Computer System Interface, an Internet Protocol -based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage...
, HyperSCSIHyperSCSIHyperSCSI was a computer network protocol for accessing storage by sending and receiving SCSI commands.It was developed by researchers at the Data Storage Institute in Singapore in 2000 to 2003....
, ATA over EthernetATA over EthernetATA over Ethernet is a network protocol developed by the Brantley Coile Company, designed for simple, high-performance access of SATA storage devices over Ethernet networks. It is used to build storage area networks with low-cost, standard technologies.- Protocol description :AoE runs on layer 2...
or InfiniBandInfiniBandInfiniBand is a switched fabric communications link used in high-performance computing and enterprise data centers. Its features include high throughput, low latency, quality of service and failover, and it is designed to be scalable...
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
In a modern enterprise architecture disk array controllers are parts of physically independent enclosureDisk enclosure
A disk enclosure is essentially a specialized chassis designed to hold and power disk drives while providing a mechanism to allow them to communicate to one or more separate computers. Drive enclosures provide power to the drives therein and convert the data sent across their native data bus into a...
s, such as disk array
Disk array
A disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.Components of a typical disk array include:...
s placed in a storage area network
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...
(SAN) or network-attached storage
Network-attached storage
Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...
(NAS) server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
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 failoverFailoverIn computing, failover is automatic switching to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active application, server, system, or network...
to another controller (transparent to computers transmitting data) - long-running operations performed without downtimeDowntimeThe term downtime is used to refer to periods when a system is unavailable.Downtime or outage duration refers to a period of time that a system fails to provide or perform its primary function...
- 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
- snapshotsSnapshot (computer storage)In computer systems, a snapshot is the state of a system at a particular point in time. The term was coined as an analogy to that in photography. It can refer to an actual copy of the state of a system or to a capability provided by certain systems....
- Business Continuance VolumeBusiness Continuance VolumeIn disk arrays, a business continuance volume, or BCV, is EMC Corporation's term for an independently addressable copy of data volume, that uses advanced mirroring technique for business continuity purposes....
s (BCV) - replicationDisk mirroringIn data storage, disk mirroring or RAID1 is the replication of logical disk volumes onto separate physical hard disks in real time to ensure continuous availability...
with a remote controller....
Simple controllers
Peripheral Component Interconnect
Conventional PCI is a computer bus for attaching hardware devices in a computer...
expansion card
Expansion card
The expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard or backplane to add functionality to a computer system via the expansion bus.One edge of the expansion card holds the contacts that fit exactly into the slot...
or just built onto a motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
. Such a controller usually provides host bus adapter
Host adapter
In computer hardware, a host controller, host adapter, or host bus adapter connects a host system to other network and storage devices...
(HBA) functionality itself to save physical space. Hence it is sometimes called a RAID adapter.
Intel started integrating their own Matrix RAID controller
Intel Matrix RAID
Intel Rapid Storage Technology is a computer storage technology marketed by Intel.It is a firmware RAID system, rather than hardware RAID or software RAID.- Description:It first appeared in the ICH6R "southbridge" chip...
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 SCSISCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
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-4 (and thus the Ultra-DMA-Mode 0
Direct memory access
Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
, 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
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
backup to finish writes interrupted by a power outage.