HPE ProLiant User Manual page 25

For any given number of drives, data loss is least likely to occur when the drives are arranged into the configuration that has the largest
possible number of parity groups. For example, four parity groups of three drives are more secure than three parity groups of four drives.
However, less data can be stored on the array with the larger number of parity groups.
All data is lost if a second drive fails in the same parity group before data from the first failed drive has finished rebuilding. A greater
percentage of array capacity is used to store redundant or parity data than with non-nested RAID methods (RAID 5, for example). A
minimum of six drives is required.
This method has the following benefits:
Higher performance than for RAID 5, especially during writes.
Better fault tolerance than either RAID 0 or RAID 5.
Up to n physical drives can fail (where n is the number of parity groups) without loss of data, as long as the failed drives are in different
parity groups.
RAID 6 RAID 6
RAID 6 protects data using double parity. With RAID 6, two different sets of parity data are used (denoted by Px,y and Qx,y in the figure),
allowing data to still be preserved if two drives fail. Each set of parity data uses a capacity equivalent to that of one of the constituent
drives. The usable capacity is C x (n - 2) where C is the drive capacity with n drives in the array.
A minimum of 4 drives is required.
Intelligent Provisioning 4.20 User Guide for HPE ProLiant and Synergy Gen11 Servers 25