RAID Storage Concept unleashed - Part 4:

Software RAID Implementations
Software RAID can be implemented in 2 ways: A] as a pure software based RAID solution, or B] as a hybrid solution that includes some hardware designed to increase performance and reduce system CPU overhead (utilization).

1) Pure Software Model – Operating System Software RAID
In this case, the RAID implementation is an application running on the host without any additional hardware. This type of software RAID uses hard disk drives which are attached to the computer system via a built-in I/O interface or a processor-less host bus adapter (HBA).
The RAID becomes active as soon as the operating system has loaded the RAID driver software. Such pure software RAID solutions often come integrated into the server OS and usually are free of additional cost for the user.
2) Hybrid Model – Hardware-based Software RAID
While this is still software RAID, the hardware helps to overcome some of the weaknesses of pure software RAID. Such solutions usually come with additional hardware (e.g. an HBA with a RAID BIOS or just a RAID BIOS integrated onto the motherboard).
The additional BIOS makes the RAID functionality available when the system is switched on, providing redundancy during boot that reduces the impact of medium errors on RAID which otherwise could lead to data corruption
or an inoperable system. In addition, most of these solutions provide a BIOS setup software which is available at system boot.
This allows for easy setup and maintenance of the RAID array without the need to install or boot an OS from hard disk or CDHost

Hardware RAID Implementations
Hardware RAID can be implemented in a variety of ways: A] as a Independent (external) RAID Controller Card, or B] as integrated AISC chip based on RAID-on-Chip technology.
1) A Independent RAID Controller Card
This is a plug-in expansion card that usually has a built-in RAID processor (I/O processor) and its own interfaces to the drives (I/O controllers). It usually plugs into PCI-X or PCIe slots of the computer system’s motherboard.
Such plug-in cards are usually the most costly, but also the most flexible and best performing RAID solutions.Here, the RAID functionality is completely independent of the host (computer system).
2) Integrated (Embedded) Hardware RAID solutions based on RAID-on-Chip (ROC) technology
In ROC solutions, the RAID processor, memory controller, host interface, I/O interfaces for hard disk drive connection, and sometimes even the memory, are all integrated into one single chip.
This chip can be integrated into the motherboard and offers hardware RAIDcapabilities with reduced cost (as just one,highly integrated ASIC, is needed).

Note: Hardware RAID controllers can be connected to PCI or PCI-X or PCI-E slots on the servers depending on the controller designs.