Four data disks and two parity disks. The first,
P = Σ Dᵢ, weights every disk by 1; the second,
Q = Σ gⁱ·Dᵢ, weights disk i by a distinct power
of the generator g = 0x02 in GF(2⁸). Fail any two disks and watch the
2×2 system solve — with the actual field arithmetic, determinant and inverse spelled out below. Fail a
third and it runs out of equations. The full derivation is in
the essay.
