3Ware 9550SXU-12 User Manual page 20

Introducing the 3ware® SAS/SATA RAID Controller
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RAID 5 is able to tolerate 1 drive failure in the unit.
Figure 4. RAID 5 Configuration Example
RAID 6
RAID 6 requires a 3ware 9650SE or 9690SA RAID controller.
RAID 6 provides greater redundancy and fault tolerance than RAID 5. It is
similar to RAID 5, but has two blocks of parity information (P+Q) distributed
across all the drives of a unit, instead of the single block of RAID 5.
Due to the two parities, a RAID 6 unit can tolerate two hard drives failing
simultaneously. This also means that a RAID 6 unit may be in two different
states at the same time. For example, one sub-unit can be degraded, while
another may be rebuilding, or one sub-unit may be initializing, while another
is verifying.
AMCC 3ware's implementation of RAID 6 requires a minimum of five
drives. Performance and storage efficiency also increase as the number of
drives increase.
3ware SAS/SATA RAID Software User Guide, Version 9.5.1