12.4

Consider the following data and parity-block arrangement on four disks:

The $B_i$ s represent data blocks; the $P_i$ s represent parity blocks. Parity block $P_i$ is the parity block for data blocks $B_{4i-3}$ to $B_{4i}$. What, if any, problem might this arrangement present?

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This arrangement has the problem that $P_i$ and $B_{4i-3}$ are on the same disk. So if that disk fails, reconstruction of $B_{4i-3}$ is not possible, since data and parity are both lost.