In an Oracle RAC database, each instance must have at least two groups of redo log files. You must allocate the redo log groups before enabling a new instance with the ALTER DATABASE ENABLE INSTANCE instance_name command. When you use DBCA to create the database, DBCA allocates redo log files to instances, as required, automatically. You can change the number of redo log groups and the size of the redo log files as required either during the initial database creation or as a post-creation step.
When the current group fills, an instance begins writing to the next log file group. If your database is in ARCHIVELOG mode, then each instance must save filled online log groups as archived redo log files that are tracked in the control file. During database recovery, all enabled instances are checked to see if recovery is needed. If you remove an instance from your Oracle RAC database, then you should disable the instance's thread of redo so that Oracle does not have to check the thread during database recovery.
Redo log management must be considered when the number of instances for a particular production Oracle RAC database changes. For example, if you increase the cardinality of a server pool for a policy-managed database and a new server is allocated to the server pool, then Oracle starts an instance on the new server. As soon as the database instance on the new server starts, it will require a set of redo log groups. Using Oracle Managed Files based on an Oracle ASM disk group, allocation of the required redo log thread and respective files is performed automatically. You should create redo log groups only if you use administrator-managed databases.
For administrator-managed databases, each instance has its own online redo log groups. Create these redo log groups and establish group members. To add a redo log group to a specific instance, specify the INSTANCE clause in the ALTER DATABASE ADD LOGFILE statement. If you do not specify the instance when adding the redo log group, then the redo log group is added to the instance to which you are currently connected.
See Also:
"About Designing and Deploying Oracle RAC Environments" for more information about administrator and policy management for databases
Oracle Database Administrator's Guide for information about creating redo log groups and establishing group members
Oracle Database SQL Language Reference for information about the ALTER DATABASE ADD LOGFILE SQL statement
Each instance must have at least two groups of redo log files. You must allocate the redo log groups before enabling a new instance with the ALTER DATABASE ENABLE INSTANCE instance_name command. When the current group fills, an instance begins writing to the next log file group. If your database is in ARCHIVELOG mode, then each instance must save filled online log groups as archived redo log files that are tracked in the control file.
During database recovery, all enabled instances are checked to see if recovery is needed. If you remove an instance from your Oracle RAC database, then you should disable the instance's thread of redo so that Oracle does not have to check the thread during database recovery.