A redo thread is a set of redo log files that an Oracle instance uses to log changes made to the database. Each instance's redo thread must contain at least two redo log groups. Each redo log group contain at least two members: a redo log and its mirrored copy. If you create your Oracle RAC database using Oracle DBCA, then your Oracle RAC database automatically implements a configuration that meets the Oracle recommendations. Oracle highly recommends to use Oracle Managed Files (OMF) on Oracle ASM.
In an Oracle RAC database, all the redo log files reside on shared storage. In addition, each instance has access to the redo log files of all the other instances in the cluster. If your Oracle RAC database uses Oracle ASM, then Oracle ASM manages the shared storage for the redo log files and the access to these files.
In an Oracle RAC database, each instance writes and archives the redo log groups in its redo thread in the same manner that single-instance Oracle AI Databases do. However, in recovery mode, the instance performing the recovery can read and process all the redo threads for the database, regardless of which instance generated the redo thread. Being able to read all the redo threads enables a running instance to recover the work completed by one or more failed instances.
For example, assume that you have an Oracle RAC database with two instances, instance A and instance B. If instance A is down, then instance B can read the redo log files for both instance A and B to ensure a successful recovery. Users can continue to access and update the database without waiting for the failed instance to be restarted.
The diagram assumes the use of Oracle ASM Diskgroups.