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.