Global Resources Coordination

Cluster operations require synchronization among all instances to control shared access to resources. 

RAC uses the Global Resource Directory (GRD) to record information about how resources are used within a cluster database. The Global Cache Services (GCS) and Global Enqueue Services (GES) manage the information in the GRD.

Each instance maintains a part of the GRD in its System Global Area (SGA). The GCS and GES nominate one instance to manage all information about a particular resource. This instance is called the resource master. Also, each instance knows which instance masters which resource.

Maintaining cache coherency is an important part of RAC activity. Cache coherency is the technique of keeping multiple copies of a block consistent between different Oracle instances. GCS implements cache coherency by using what is called the Cache Fusion algorithm.

The GES manages all non–Cache Fusion interinstance resource operations and tracks the status of all Oracle enqueuing mechanisms. The primary resources of the GES controls are dictionary cache locks and library cache locks. The GES also performs deadlock detection to all deadlock-sensitive enqueues and resources. 


Global Resources Coordination Example
Write to Disk coordination Example