Hi,
If you are talking about clustering option as it was in 5.6.1 then as far as i understood, the default installation of AEM6 comes with CRX3 which does not support clustering. If you want clustering you need to either install using CRX2 or use MongoMK. CRX3 is highly optimized for single node implementation. If you want to use clusterting for reliability you can follow the method mentioned into below slide share deck
http://www.slideshare.net/jukka/the-new-repository-in-aem-6
If you want both reliability and scalability then MongoMK is the way to go in new AEM 6.0.
Refer following link for clustering info related to OAK
http://jackrabbit.apache.org/oak/docs/clustering.html
Looking for other experts comments on the forum to correct me if this is wrong understanding.
Thanks
Deepak