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Public and Private Rollback Segments

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%.

Rollback segments can be either public or private. Private rollback segments are used exclusively by an instance. Public rollback segments can be acquired by any of the instances. They are exactly the same in their functionality.

A public rollback segment is created by the PUBLIC clause with the create ROLLBACK SEGMENT command. Any rollback segment created without the PUBLIC clause is considered a private rollback segment. As mentioned, public rollback segments can be acquired by any instance, while private rollback segments are acquired by a specific instance only when the rollback segment?s name has been listed in the rollback_segment initialization parameter for the instance, as shown here:


It is preferable to use private rollback segments and they are recommended for ease of administration. When private rollback segments are used, it is easy to determine which rollback segments are in use by which instance. This facilitates planning in advance based on the workload for a typical instance. The following example shows how to create a public rollback segment:

create public rollback segment rbseg1

tablespace rbs11 ;

Concept of Thread

In the RAC system, each instance has to have its own redo log groups. The redo log file groups of an instance are collectively called a thread, or more appropriately, a redo log thread. Each instance has its own redo thread. The redo log groups function in a true circular fashion; as one fills up, another one records the redo entries. In a stand-alone instance, there is only one thread. In a RAC system, typically there are as many threads as instances. The thread number identifies each thread. The threads may have different numbers of redo groups, but each group must have at least two members, as shown in Figure 8.1.

Figure 8.1: Redo Log threads

Online redo logs record the redo entries as transactions commit and rollback.  Redo groups may optionally have additional members to provide mirroring of the redo groups.

Thread Features

Each instance must have a minimum of two redo groups, with each group having at least one member in the group. Every redo group has a group number, which is a unique number in the database. All the redo log files supporting the redo groups reside on shared storage, so that every instance in the cluster can access all the redo groups during the recovery process. As shown in Figure 8.2, all the redo groups are located on a shared storage unit.

Figure 8.2: Redo Groups on a shared storage

Use a minimum of three or more redo groups in a thread. Keep at least two redo members for each redo group for multiplexing and protection. Multiplexing the redo members is optional but is highly recommended. Different degrees of mirroring are permitted in different threads.

The redo log thread can be public or private. When the redo thread is enabled, it can be specified as a private thread. This means it is associated with a specific instance as defined in the initialization parameter thread of the instance. Alternatively, it can be specified as public, in which case any instance can acquire it.


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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