A RAID Controller is a hardware device that is used to present a number of physical hard disk drives as logical units.
Hardware RAID Controllers are normally PCI or PCI Express (PCIe) cards and these cards are designed to support specific drive formats such as SATA, SCSI or IDE. Some cards will support a number of different formats. As well as expansion cards, some RAID Controllers are integrated with the Motherboard itself. When first introduced in the late 1980s, these controllers revolutionised the way data was stored and quickly became very popular.
The term RAID is an acronym for Redundant Array raid controller of Inexpensive Disks and the main reason for using a RAID system is redundancy, by storing data in multiple locations across multiple physical disks.
There are a number of RAID levels that can be used, with the main levels being RAID 0 and RAID 1. Level 0 is where the data is distributed evenly across two or more physical disks and does not provide any parity information for redundancy, but it can provide better performance. Level 1 is where multiple disks have a mirror copy of the same data, thus providing the redundancy and potentially better performance when reading the data.
Other RAID levels are designed to provide higher levels of redundancy, greater performance when retrieving the data and more sophisticated error protection.
Why use a RAID System and what should you hope to gain from such a system? Fault Tolerance is probably one of the main benefits, due to the fact that a single hard drive failure will not result in loss of all the data, and depending on the RAID implementation and level of redundancy, maybe no data loss at all except in extremely exceptional circumstances. This means that the security of the data is also a benefit of using RAID. Performance can be greatly enhanced because the RAID Controller is able to access multiple copies of the data on multiple physical devices. Using multiple disks can greatly increase the storage capacity, but bear in mind that the RAID implementation used will reduce the overall capacity, depending on the level of redundancy.
If you are ordering a RAID card then you have to make sure that the card you order is designed to operate with the particular hard disk type you are using. In other words, if you have SCSI hard disks then an IDE RAID controller card is not designed for this purpose.