RAID 6 extends RAID 5 by using block-level striping with two parity blocks, instead of one, distributed across all member disks.
RAID 6 requires at least 4 drives to implement.
Characteristics & Advantages
Very fast Read data transfer rate
Extremely high data fault tolerance
Can sustain two drive failures because of the two parity checks, data is still accessable.
Disadvantages
Transaction rate equal to a single disk drive at best (if spindles are synchronized)
Parity calculation overhead slows down the Write data transaction rate
Requires n+2 drives to implement because of dual parity scheme