GRD stands for Global Resource Directory. The GES and GCS maintains records of the statuses of each datafile and each cahed block using global resource directory.This process is referred to as cache fusion and helps in data integrity.

Oracle RAC is composed of two or more instances. When a block of data is read from datafile by an instance within the cluster and another instance is in need of the same block,it is easy to get the block image from the instance which has the block in its SGA rather than reading from the disk. To enable inter instance communication Oracle RAC makes use of interconnects. The Global Enqueue Service(GES) monitors and Instance enqueue process manages the cahce fusion.

To Understand => The GCS and GES itself is not a process. It is implemented by the LMDs,LMSs and other RAC background processes coordinating with one another on multiple nodes to handle GLOBAL Resources.

The GCS and GES maintain a Global Resource Directory to record information about resources. The Global Resource Directory resides in memory, is distributed throughout the cluster, and is available to all active instances. In this distributed architecture, each node participates in the management of information in the directory. This distributed scheme provides fault tolerance and enhanced runtime performance.

The GCS and GES ensure the integrity of the Global Resource Directory even if multiple nodes fail. The shared database is always accessible if at least one instance is active after recovery is completed. The fault tolerance of the resource directory also enables Real Application Clusters instances to start and stop at any time, in any order.