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Online redo logs
Redo log groups and members
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.
Path and name of redo logs
select member from v$logfile;
Size of Online Redo Log
The smaller the online redo logs are, the less time it takes to start up a database in a instance recovery. However, if redo logs are small, the database must perform more checkpoints which is I/O resource intensive. If the sizes are too small, this will result in 'slow' checkpointss (Message in alert.log: Thread .. cannot allocate new log...)
Active, inactive and current redo logs
A log group can have different states, unused, current, active, clearing, clearing_current and inactive. This state can be queried with
select status from v$log
Inactive redo are not needed for instance recovery. Most probably, an inactive redo log has been archived.
Recovering database when redo log is lost
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
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
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
Renaming a redo log group or member is a structural change to the database that requires a backup.
Backing up the online redo logs
The online redo logs should never be backed up. Therefore, they're (usually) multiplexed.