RAID Controllers Information
RAID controllers are computer boards that control RAID systems. RAID is an acronym for Redundant Array of Independent (or Inexpensive) Disks. A RAID system is a group of drives which together act as a single storage system. RAID is a performance enhancing method of storing the same data in different places on multiple hard disks. RAID controllers contain and manage the guidelines or protocols to achieve this task. There are many classifications of RAID. Commonly used RAID types include RAID 0 (Data Striping), RAID 1 (Disk Mirroring) and RAID 5 controller (Striping with Distributed Parity).
RAID controllers are available in two main configurations: as busses or controller cards and external peripheral devices. Bus-based or controller card hardware RAID is a conventional type of hardware RAID, and is most commonly used for lower-end systems. This specialized RAID controller card (computer board) is installed into the PC or server, and the array drives are connected to it. It essentially takes the place of the SCSI host adapter or IDE/ATA controller that would normally be used for interfacing between the system and the hard disks; it interfaces to the drives using SCSI or IDE/ATA, and sends data to the rest of the PC over the system bus (typically PCI). These devices are often referred to as SCSI Raid controllers, SCSI host RAID controllers or PCI RAID controllers. Some motherboards, particularly those intended for server systems, come with some variant of integrated RAID controller driver. These are built into the motherboard, but function in precisely the same manner as an add-in bus-based card. The only difference is that integrated controllers can reduce overall cost at the price of flexibility.
External RAID Controllers
External RAID controllers are standalone box type devices for use with higher-end design systems. In this case the RAID controller is removed completely from the system to a separate box. Unlike RAID controller cards, they are not mounted within the card cage or motherboard. Within the box the RAID controller manages the drives in the array, typically using SCSI, and then presents the logical drives of the array over a standard interface (again, typically a variant of SCSI) to the server using the array. The server sees the array or arrays as just one or more very fast hard disks; the RAID is completely hidden from the machine. In essence, one of these units really is an entire computer unto itself, with a dedicated processor that manages the RAID array and acts as a conduit between the server and the array.