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Oracle 11.2 fully supports continuing to use non-11.2 databases on a cluster that has been upgraded to 11.2. However, you should be aware that the Oracle 11.2.0.1 base release contained a number of problems that could affect users with a 10.2 database. Known problem with brand new installations of Grid Infrastructure 11.2 and pre-11.2 RDBMS instances is related to node pinning. During the upgrade, the nodes containing 10.2 RDBMS software will be pinned, allowing pre-11.2 databases to run on them. Important aspect is with Non-upgraded clusters are not automatically pinned, which causes problems with emca and dbca when executed from the pre-11.2 Oracle homes. You should use the utilities from the $GRID_HOME to manage resources provided by the same Oracle home. Similarly, when managing pre-11.2 resources, you can use the commands from their respective homes. This is a golden rule to remember You can find more information about problems with 10.2 databases and 11.2 Grid Infras...

In 11gR2, Oracle has introduced a new registry to maintain the clusterware resources (css, crs,evm,gip and more) in a new registry called Oracle Local Registry (OLR) . Multiple processes on each node have simultaneous read and write access to the OLR particular to the node on which they reside, regardless of whether Oracle Clusterware is running or fully functional. By default, OLR is located at Grid_home/cdata/host_name.olr on each node.The OCR still exists, but maintains only the cluster resources. Until Oracle Database 11gR1, the RAC configurations consisted of just one registry when running Oracle Clusterware. Shortly called OCR, Oracle Cluster Registry, maintained the cluster level resource information, privileges etc. To be precise, the OCR maintained information about 2 sets of node level resources, namely, the Oracle Clusterware Components (CRS, CSS, evm) as well as Cluster resources (DB, Listener etc).