NetApp Storage RAID Group Recommendations

RAID-DP is the recommended RAID type to use for all NetApp storage configurations. In Data ONTAP 8.0.1, the maximum SATA RAID group size for RAID-DP has increased from 16 to 20.

For HDD (SATA, FC, and SAS) the recommended sizing approach is to establish a RAID group size that is within the range of 12 (10+2) to 20 (18+2); that achieves an even RAID group layout (all RAID groups contain the same number of drives). If multiple RAID group sizes achieve an even RAID group layout, NetApp recommends using the higher RAID group size value within the range. If drive deficiencies are unavoidable, as is sometimes the case, NetApp recommends that the aggregate should not be deficient by more than a number of drives equal to one less than the number of RAID groups. Otherwise you would just pick the next lowest RAID group size. Drive deficiencies should be distributed across RAID groups so that no single RAID group is deficient more than a single drive.

Given the added reliability of SAS and Fibre Channel (FC) drives, it might sometimes be justified to use a RAID group size that is as large as 24 (22+2) if this aligns better with physical drive count and storage shelf layout.
SSD is slightly different. The default RAID group size for SSD is 23 (21+2), and the maximum size is 28. For SSD aggregates and RAID groups, NetApp recommends using the largest RAID group size in the range of 20 (18+2) to 28 (26+2) that affords the most even RAID group layout, as with the HDD sizing approach.