Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 8 - RAC Administration

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 records the redo entries. In a stand-alone instance, there is only one thread. In a RAC system, typically you have 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 Fig 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 a shared storage, so that every instance in the cluster can access all the redo groups during the recovery process. As shown in Fig 8.2, all the redo groups are located on a shared storage unit.