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Concept of Redo Thread

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%.

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 redo log 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 4.7.

Figure 4.7:  Redo Threads in a 2 Node RAC database 

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 4.8, all the redo groups are located on a shared storage unit.

Figure 4.8: Redo Groups on a shared storage

Use a minimum of three 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 and any instance can acquire it.

Database Logical Objects

The logical storage structures include, at the lowest level, the data blocks and next level the extents. A group of extents set aside for a specific object are grouped into a segment. At the highest level, a tablespace which consist of extents will be found.

Data Blocks

The Oracle data rows are stored in Data Blocks. The standard size of the data block is specified by the db_block_size initialization parameter. In addition up to five other block sizes can be specified. A Data Block is the smallest or most granular basic logical structure that is brought into the buffer cache from disk storage in order to do SQL processing.


The next level of logical database structure is an extent. An extent is a specific number of contiguous data blocks, which are obtained in a single allocation unit. It is an allocation unit.


A Segment is a set of extents allocated for a certain logical structure. There are many types of segments namely, data segment, index segments, temporary segments, and undo segments.


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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