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7.2 RAID-6


This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.

7.2 RAID-6

    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


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