
There are 10 standard RAID levels, 0 through 9. RAID levels describe the configuration of a RAID array, and relate to the patterns in which data is written to the RAID drive, the fault tolerance of the RAID array, and the space efficiency. To some degree, the RAID level can affect the number of drives in the array, but that doesn't mean that, for instance, RAID 9 uses nine hard drives. You can run any RAID level up to 9 with just two drives.
Hybrid RAID Levels
Beyond the standard RAID levels are Hybrid RAID levels, which means two different RAID levels are “nested” within one controller. (Rarely are more than two RAID levels nested.) Hybrid RAID array configurations include designations such as RAID 0+1, RAID 1+0, or RAID 5+0. Some people may eliminate the plus sign, resulting in RAID 10 or RAID 50, but that doesn't mean RAID “ten” or RAID “fifty.”