This figure illustrates three threads of redo for three Real Application Clusters instances, instances X, Y, and Z.

Instance X has three redo log groups, group one, group two and group three. Each redo log group is configured with two redo log group members, that is, two copies of the redo logs. This is known as "mirroring."

In instance X redo log group one writes to redo log group two and redo log group two writes to redo log group three. Redo log group three writes to redo log group one.

Instance Y has two redo log groups labelled as group four and group five. Both redo log groups are configured with three redo log group members each, that is, three mirrored copies of the redo logs.

In instance Y redo log group four writes to redo log group five and redo log group five writes to redo log group four.

Instance Z has three redo log groups, labelled as group six, group seven, and group eight. All the log groups are configured with only one group member. Therefore, these groups are not mirrored.

In instance Z redo log group six writes to redo log group seven and redo log group seven writes to redo log group eight, and redo log group eight writes to redo log group six.