Raid 6 - Hitachi HA800 Series User Manual

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 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.
F10 mode options 18