IBM N series Hardware Manual page 181

When the command is entered, the aggregate or traditional volumes (as in the following
examples) are instantly denoted as RAID-DP. However, all diagonal parity stripes still must be
calculated and stored on the second parity disk. Figure 10-21 shows the use of the command
to convert the volume.
Figure 10-21 The itso volume conversion from traditional RAID4 to RAID-DP
As shown in this example, when the raid attribute for the volume itso is changed, it is changed
to RAID DP for all volumes within the aggregate. Protection against double disk failure is not
available until all diagonal parity stripes are calculated and stored on the diagonal parity disk.
Figure 10-22 shows a "reconstruct" status that signifies that diagonal parity creation in
progress.
Figure 10-22 The itso volume in reconstruct status during conversion of diagonal parity RAID-DP
Calculating the diagonals as part of a conversion to RAID-DP takes time and affects
performance slightly on the storage controller. The amount of time and performance effect for
conversions to RAID-DP depends on the storage controller and how busy the storage
controller is during the conversion. Run conversions to RAID-DP during off-peak hours to
minimize potential performance effect to business or users.
For conversions from RAID4 to RAID-DP, certain conditions are required. Conversions at the
aggregate or traditional volume level require an available disk for the second diagonal parity
disk for each RAID4 group. The size of the disks that are used for diagonal parity must be at
least the size of the original RAID4 row parity disks. In the example, the volume itso is
altered from an RAID4 status to RAID-DP.
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Chapter 10. Data protection with RAID Double Parity