Tuesday, April 29, 2008


Diagram of a RAID 6 setup which is just like RAID 5 but with two parity blocks instead of one

A RAID 6 extends RAID 5 by adding an additional parity block, thus it uses block-level striping with two parity blocks distributed across all member disks. It was not one of the original RAID levels.

RAID 5 can be seen as a special case of a Reed-Solomon code
[3]. RAID , being a degenerate case, requires only addition in the Galois field. Since we are operating on bits, the field used is a binary galois field
. In cyclic representations of binary galois fields, addition is computed by a simple

After understanding RAID 5 as a special case of a Reed-Solomon code,
it is easy to see that it is possible to extend the approach to produce
redundancy simply by producing another syndrome; typically a polynomial
(8 means we are operating on bytes). By adding additional syndromes it
is possible to achieve any number of redundant disks, and recover from
the failure of that many drives anywhere in the array, but RAID 6
refers to the specific case of two syndromes.

Like RAID 5, the parity is distributed in stripes, with the parity blocks in a different place in each stripe.
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