IBM N Series Hardware Manual page 159

The algorithm continues determining whether additional diagonal blocks can be re-created.
The upper left block is re-created from row parity, and RAID-DP can proceed in re-creating
the gray diagonal block in column two, row two. See Figure 10-13.
Figure 10-13 RAID-DP reconstruction simulation of gray block column two
RAID-DP recovers the gray diagonal block in column two, row two. Adequate information is
now available for row parity to re-creating the missing horizontal white block (one) in the first
column, row two (Figure 10-14).
Figure 10-14 RAID-DP reconstruction simulation of white block column one
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Chapter 10. Data protection with RAID Double Parity