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Global Cache Service (GCS)

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

GCS is the main controlling process that implements Cache Fusion. GCS tracks the location and the status (mode and role) of the data blocks, as well as the access privileges of various instances. GCS is the mechanism which guarantees the data integrity by employing global access levels. GCS maintains the block modes for data blocks in the global role. It is also responsible for block transfers between the instances. Upon a request from an Instance GCS organizes the block shipping and appropriate lock mode conversions. The Global Cache Service is implemented by various background processes, such as the Global Cache Service Processes (LMSn) and Global Enqueue Service Daemon (LMD).

Global Enqueue Service

The Global Enqueue Service (GES) manages or tracks the status of all the Oracle enqueuing mechanism. This involves all non Cache fusion intra-instance operations. GES performs concurrency control on dictionary cache locks, library cache locks, and the transactions. GES does this operation for resources that are accessed by more than one instance.


GES and GCS have the memory structures associated with global resources. It is distributed across all instances in a cluster. This area is located in the variable or shared pool section of the SGA. As an example, below list shows the additions.

POOL         NAME                            BYTES
------------ -------------------------- ----------
shared pool  gcs shadows                  12143272
shared pool  gcs resources                17402792
shared pool  ges enqueues                  4079516
shared pool  ges resources                 2624824
shared pool  ges big msg buffers           3839044

So far all the memory structures and background processes have been surveyed. Now attention will be turned to the physical database structures of the database which includes the data files, redo log files and control files among other type of files.

Database Related Files

Oracle Database is a collection of physical files. These are basically the operating system files used by the database and database instance. Oracle RAC needs shared storage to store the files. RAC follows the shared disk model, where all the cluster nodes share the same disk or storage volumes.

The files included in the RAC architecture are shown in Figure 4.6. Most of them must be available simultaneously and be updateable by all the nodes, and hence, by all the instances in the cluster. Additionally, there are some files which can remain on local file system.

Figure 4.6: Database related files 

The following is a description of each of the categories of files.

Data Files

These are the main files of the database. The data files contain the actual data. The logical database structures like tables and indexes are physically stored in the data files. These files are located on shared storage and are accessible by all the nodes in the cluster. One or more data files form a logical unit of database storage which is called the tablespace.

Data Files can be associated with only one database. By locating the data files either on a clustered file system or raw partition they are made accessible by all the nodes. This is a minimum requirement in the RAC database system.

Control Files

The control files contain entries that specify the physical structure of the database. The control files contain the key information such as the database name, name and location of the data files, and redo log files for the database.

When an instance is launched, the control files identify the data and redo log files. Control Files are usually multiplexed and they are located on the shared storage either on a clustered file system or raw partition


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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