sentences:  Nested RAID levels
  
  Nested RAID levels, also known as hybrid RAID, combine two or more of the standard RAID levels (where "RAID" stands for "redundant array of independent disks") to gain performance, additional redundancy or both, as a result of combining properties of different standard RAID layouts.Nested RAID levels are usually numbered using a series of numbers, where the most commonly used levels use two numbers.