There are recommendations to keep in mind when choosing what kind of volume to use for the root volume.
If a root volume exists as a traditional volume, it can be a stand-alone RAID4 or RAID-DP volume. RAID4 requires a minimum of two disks and can protect against single-disk failures. RAID-DP, the default RAID type, requires a minimum of three disks and can protect against double-disk failures. Using RAID-DP for the root aggregate is recommended.
Data ONTAP 8.0 or later allows you to create only a new FlexVol root volume, not a new traditional root volume, from the boot menu. However, preexisting traditional root volumes are still supported.
For information about RAID types, traditional volumes, and FlexVol volumes, see the Data ONTAP Storage Management Guide for 7-Mode.
However, for small storage systems where cost concerns outweigh resiliency, a FlexVol based root volume on a regular aggregate might be more appropriate.
For a V-Series system that does not have a disk shelf, the root volume resides on the third-party storage. You can install only one root volume per V-Series system, regardless of the number of storage arrays or disk shelves that the V-Series system uses for storage.
FlexVol recovery commands work at the aggregate level, so all of the aggregate's disks are targeted by the operation. One way to mitigate this effect is to use a smaller aggregate with only a few disks to house the FlexVol volume containing the root volume.
For Data ONTAP 7.3 and later, the default RAID type for traditional root volume is RAID-DP. If you want to use RAID4 as the raid type for your traditional root volume to minimize the number of disks required, you can change the RAID type from RAID-DP to RAID4 by using vol options vol0 raidtype raid4.