Online redo logs

An online redo log group is a group of (possibly one) online redo log files. Each of these files in the groups is called an online redo log member of the group. LGWR writes to all members of the active group and never to a member of a group that is not active. Exactly one group is active at a time. A group becomes inactive and another active through a redo log switch.

The purpose of having multiple members in a group is to ensure that if a redo log file becomes unusable another file of the unusable file's redo log group can be used instead.

select member from v$logfile;

The smaller the online redo logs are, the less time it takes to start up a database in a instance recovery. However, if redo logs are small, the database must perform more checkpoints which is I/O resource intensive. If the sizes are too small, this will result in 'slow' checkpointss (Message in alert.log: Thread .. cannot allocate new log...)

A log group can have different states, unused, current, active, clearing, clearing_current and inactive. This state can be queried with

select status from v$log

Inactive redo are not needed for instance recovery. Most probably, an inactive redo log has been archived.

If the redo log is lost, the database can incompletely be recovered, if the database is in archive log mode usually with a

recover database until cancel;

Of course, the commited transactions that 'went' into the last current online log are lost, but it doesn't prevent recovery.

Adding a redo log group or member is a structural change to the database that requires a backup.

Dropping a redo log group or member is a structural change to the database that requires a backup.

Renaming a redo log group or member is a structural change to the database that requires a backup.

The online redo logs should never be backed up. Therefore, they're (usually) multiplexed.