9.3. Hardware RAID versus Software RAID

There are two possible RAID approaches: Hardware RAID and Software RAID.

9.3.1. Hardware RAID

The hardware-based array manages the RAID subsystem independently from the host and presents to the host only a single disk per RAID array.
An example of a Hardware RAID device would be one that connects to a SCSI controller and presents the RAID arrays as a single SCSI drive. An external RAID system moves all RAID handling "intelligence" into a controller located in the external disk subsystem. The whole subsystem is connected to the host via a normal SCSI controller and appears to the host as a single disk.
RAID controllers also come in the form of cards that act like a SCSI controller to the operating system but handle all of the actual drive communications themselves. In these cases, you plug the drives into the RAID controller just like you would a SCSI controller, but then you add them to the RAID controller's configuration, and the operating system never knows the difference.