IBM N Series, Hardware Manual

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IBM System Storage N series Hardware Guide
With this in mind, traverse the blue series diagonal blocks in Figure 10-10 on page 155.
Notice that only one of the five blue series blocks are missing. With four out of five elements
available, RAID-DP has all of the information that is needed to reconstruct the data in the
missing blue series block. Figure 10-11 shows that this data is recovered over to an available
hot spare disk.
Figure 10-11 RAID-DP reconstruction simulation diagonal blue block
The data was re-created from the missing diagonal blue block by using the same arithmetic
that was previously described (12 - 7 - 2 - 2 = 1). Now that the missing blue series diagonal
information is re-created, the recovery process switches from using diagonal parity to using
horizontal row parity. Specifically, the top row after the blue block re-creates the missing
diagonal block. There is now enough information available to reconstruct the single missing
horizontal gray block in column 1, row 1, disk 3 parity (9 - 3 - 2 - 1 = 3). This process is shown
in Figure 10-12.
Figure 10-12 RAID-DP reconstruction of first horizontal block