RAID Implementations

The distribution of data across multiple disks can be managed by either dedicated hardware or by software. Additionally, there are hybrid RAIDs that are partly software- and partly hardware-based solutions.
With a software implementation, the operating system manages the disks of the array through the normal drive controller (IDE/ATA, SATA, SCSI, Fibre Channel, etc.). With present CPU speeds, software RAID can be faster than hardware RAID[citation needed], though at the cost of using CPU power which might be best used for other tasks. One major exception is where the hardware implementation of RAID incorporates a battery backed-up write back cache which can speed up an application, such as an OLTP database server. In this case, the hardware RAID implementation flushes the write cache to secure storage to preserve data at a known point if there is a crash. The hardware approach is faster than accessing the disk drive and limited by RAM speeds, the rate at which the cache can be mirrored to another controller, the amount of cache and how fast it can flush the cache to disk. For this reason, battery-backed caching disk controllers are often recommended for high transaction rate database servers. In the same situation, the software solution is limited to no more flushes than the number of rotations or seeks per second of the drives. Another disadvantage of a pure software RAID is that, depending on the disk that fails and the boot arrangements in use, the computer may not be able to be rebooted until the array has been rebuilt.
A hardware implementation of RAID requires at a minimum a special-purpose RAID controller. On a desktop system, this may be a PCI expansion card, or might be a capability built in to the motherboard. In larger RAIDs, the controller and disks are usually housed in an external multi-bay enclosure. The disks may be IDE, ATA, SATA, SCSI, Fibre Channel, or any combination thereof. The controller links to the host computer(s) with one or more high-speed SCSI, Fibre Channel or iSCSI connections, either directly, or through a fabric, or is accessed as network attached storage. This controller handles the management of the disks, and performs parity calculations (needed for many RAID levels). This option tends to provide better performance, and makes operating system support easier. Hardware implementations also typically support hot swapping, allowing failed drives to be replaced while the system is running. In rare cases hardware controllers have become faulty, which can result in data loss. Hybrid RAIDs have become very popular with the introduction of inexpensive hardware RAID controllers. The hardware is a normal disk controller that has no RAID features, but there is a boot-time application that allows users to set up RAIDs that are controlled via the BIOS. When any modern operating system is used, it will need specialized RAID drivers that will make the array look like a single block device. Since these controllers actually do all calculations in software, not hardware, they are often called "fakeraids". Unlike software RAID, these "fakeraids" typically cannot span multiple controllers.
Both hardware and software versions may support the use of a hot spare, a preinstalled drive which is used to immediately (and almost always automatically) replace a drive that has failed. This reduces the mean time to repair period during which a second drive failure in the same RAID redundancy group can result in loss of data.
Some software RAID systems allow one to build arrays from partitions instead of whole disks. Unlike Matrix RAID they are not limited to just RAID 0 and RAID 1 and not all partitions need to be RAID.

Jagath Krishnakumar