Recommendations and requirements for Extended Clusters for RAC (EC RAC) configurations with Oracle RAC 10g or 11g

Oracle 10g Release 2 and later supports up to two copies of the Oracle Cluster Registry (OCR) and up to three vote disks. For EDC, each copy of OCR and each vote disk are required to be physically mirrored between the two datacenters. The mirrored OCR and vote disks ensure that Oracle Clusterware has access to local physical copies for Oracle Clusterware cluster reformation.

For ER RAC configurations, it is recommended to maintain local storage for Oracle Clusterware and Oracle Database binaries and HOME, to reduce inter-site traffic.