Oracle Database 10g contains many enhancements, but one of the most interesting is the introduction of Oracle Cluster Ready Services (CRS), Oracle's full-stack clusterware.
Oracle CRS is Oracle's own clusterware tightly coupled with Oracle Real Application Clusters (RAC).
CRS must be installed prior to the installation of Oracle RAC.
It can also work over any third-party clustering software but there is no longer a requirement to buy and deploy such software.
In short, Oracle CRS is primarily responsible for managing the high-availability (HA) architecture of Oracle RAC with the help of Cluster Ready Services Daemon (CRSD), Oracle Cluster Synchronization Server Daemon (OCSSD) and the Event Manager Daemon (EVMD).
The CRSD manages the HA functionality by starting, stopping, and failing over the application resources and maintaining the profiles and current states in the Oracle Cluster Registry (OCR).
The OCSSD manages the participating nodes in the cluster by using the voting disk. The OCSSD also protects against the data corruption potentially caused by "split brain" syndrome by forcing a machine to reboot.
Although Oracle CRS replaces the Oracle Cluster Manager (ORACM) in Oracle9i RAC, it does continue support for the Global Services Daemon (GSD), which in Oracle9i is responsible for communicating with the Oracle RAC database.
In Oracle 10g, GSD's sole purpose is to serve Oracle9i clients (such as SRVCTL, Database Configuration Assistant, and Oracle Enterprise Manager).
Source :-
http://download.oracle.com/docs/html/B10766_08/toc.htm
