Oracle Cluster Registry-OCR

• Cluster configuration information is maintained in the OCR. 
• The OCR relies on distributed shared cache architecture for optimizing queries, and clusterwide atomic updates against the cluster registry. 
• Each node in the cluster maintains an in-memory copy of OCR, along with the CRSD that accesses its OCR cache. Only one of the CRSD processes actually reads from and writes to the OCR file on shared storage. This process is responsible for refreshing its own local cache, as well as the OCR cache on other nodes in the cluster. 
• For queries against the cluster registry, the OCR clients communicate directly with the local CRS daemon (CRSD) process on the node from which they originate. When clients need to update the OCR, they communicate through their local CRSD process to the CRSD process that is performing input/output (I/O) for writing to the registry on disk. 
• The main OCR client applications are OUI, SRVCTL, Enterprise Manager (EM), the Database Configuration Assistant (DBCA), the Database Upgrade Assistant (DBUA), Network Configuration Assistant (NETCA), and the ASM Configuration Assistant (ASMCA). 
• The installation process for Oracle Clusterware gives you the option of automatically mirroring OCR. This creates a second OCR file, which is called the OCR mirror file, to duplicate the original OCR file, which is called the primary OCR file. Although it is recommended to mirror your OCR, you are not forced to do it during installation. 

The Oracle Grid Infrastructure installation defines three locations for the OCR and supports up to five. New installations to raw devices are no longer supported.