Concept
A disk array controller is a device that 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. Disk array controller is often improperly shortened to disk controller. The two should not be confused as they provide very different functionality. A disk array controller provides front-end interfaces and back-end interfaces. The back-end interface communicates with the controlled disks. Hence, its protocol is usually ATA (a.k.a. PATA), SATA, SCSI, FC or SAS. The 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 dedicated protocols for specific solutions: 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. Disk array In a modern enterprise architecture disk array controllers (sometimes also called storage processors, or SPs) are parts of physically independent enclosures, such as disk arrays placed in a storage area network (SAN) or network-attached storage (NAS) servers. 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 Snapshots Business continuance volumes (BCV) Replication with a remote controller.