Minimum number of array LUNs required per ONTAP system
The number of array LUNs that you need per ONTAP system depends on the location of the root volume.
The root volume can be either on a disk or on array LUNs. The location of the root volume then determines the minimum number of array LUNs you need. If the root volume is on a storage array, each stand-alone ONTAP system and each node in an HA pair must own at least one array LUN. If the root volume is on a native disk, the only array LUNs needed are those for data storage.
For a MetroCluster configuration that is using array LUNs, two array LUNs are required (one LUN from each site) if the root volume is on a storage array. The two LUNs are necessary so that the root volume can be mirrored.