This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.
A two-dimensional disk array parity scheme was described by Randy Katz, Garth Gibson, and David Patterson (all then with UC Berkeley - Gibson is now a professor at Carnegie Mellon University) at the 1989 IEEE Compcon conference. This method had one parity calculated along the disk strings and another calculated across them. This would increase the mean-time-to-data-loss by more than 10,000 fold. I am not aware of any implementations of this configuration. Storage Technology Corp (STK - Louisville, Colorado) has described a somewhat similar scheme for their long-delayed Iceberg disk array. This would have a regular, orthogonal RAID 5 parity across drives along with a Reed-Solomon encoding on another drive. This is sometimes referred to as RAID 6 or RAID 5+. STK claims their design will allow failure of ANY TWO drives - which is beyond the survival capabilities of standard RAID 5. A RAID 5 which is 'deep' can survive failures in more than one drive so long as it doesn't lose more than one drive per rank: HBA1 HBA2 HBA3 HBA4 HBA5 HBA6 HBA7 HBA8 | | | | | | | | Rank1 Disk1 Disk2 Disk3 Disk4 Disk5 Disk6 Disk7 Disk8 | | | | | | | | Rank2 Disk9 Disk10 Disk11 Disk12 Disk13 Disk14 Disk15 Disk16 . . . . . - etc. Rank4 . . . . Disk32 If the above is a RAID 5 then losing drives 5 & 6 will destroy data. If it is a RAID 6 then it will not. Losing drives 3 and 12 will not disable a RAID 5 nor a RAID 6. But RAID 6 will cost more and may have slower performance for small random writes from having to update more parity data. I think there are clearly ways to mitigate the parity update perfomance for RAID 6 as well as RAID 5. -- Dick Wilmot Editor, Independent RAID Report (510) 938-7425