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Oracle: redo log file group

Oracle requires at least two redo log file groups.

Each group can have a different number of members. However, Oracle recommends that each group has the same number of members.

When the current redo log file group is filled up, a log switch is necessary.

Although a log file group might consist of multiple files, Oracle's documentation usually refers to a group as a log file. This makes sense given that each member's content in a group is (at least in theory) identical.

Log group status

`UNUSED`	1) The group was never written to. 2) After a `RESETLOGS` for non-current groups.
`CURRENT`	The group that is written to by LGWR. A current group is also active.
`ACTIVE`	A group that may be used for crash recovery
`CLEARING`	Log is being cleared after an `alter database clear logfile` statement
`CLEARING_CURRENT`
`INACTIVE`	Log is no longer needed for instance recovery. It might be used for media recovery.

SQL statements

alter database add logfile member '/u01/logs/MY_DB/redo02.log' to group 2;

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