Following are a list of guidelines regarding zones and clusters on the Database Domains:
When you first install the operating system instances on a domain, that domain is automatically designated as the global zone. Any domains or zones that you create from that point forward are nonglobal zones.
Global zones and nonglobal zones cannot run RAC clusterware in the same domain. When a Database Domain is created and an operating system is installed on the Database Domain, that domain is considered a global zone. The following scenarios describe what is and is not acceptable in that situation:
If you have two Database Domains (global zones) and those Database Domains are members of a cluster, you cannot create zones on those Database Domains and have those zones as members of their own clusters. Because those zones would be considered non-global zones, you cannot have those non-global zones running RAC clusterware within Database Domains that are also running RAC clusterware.
If you have two Database Domains (global zones) but those Database Domains are not members of a cluster, then you can create zones on those Database Domains and have those zones as members of their own clusters.
You can create clusters with zones of different sizes. However, each zone that is a member of a cluster should be the same size with regards to the number of cores being used by each zone in the cluster. For example, if you are creating a two-node cluster and the first zone in that cluster uses four cores, the other zone in that cluster should also use four cores.
You can cluster zones together across multiple Database Domains that are of different sizes. For example, you can create a two-node cluster, where there is one zone on the Database Domains on the two PDomains, and those zones are clustered together, even if one of the Database Domains has one CMIOU associated with it and the other Database Domain has two CMIOUs associated with it. The private interface connections (the IB connections) are limited to the lower number from the smaller domain, based on the IB HCAs available to the zone.
While it is possible to create a cluster with zones in the same Database Domain or on the same PDomain, that configuration is not a highly-available configuration. Instead, create clusters where the zones are on separate Database Domains, and the cluster spans across PDomains, so that there is not a single point of failure.