CURRENT - This is the current redo log. This implies that the redo log is active. The redo log could be open or closed.
ACTIVE - The log is active but is not the current log. It is needed for crash recovery. It may be in use for block recovery. It might or might not be archived.
CLEARING - The log is being re-created as an empty log after an ALTER DATABASE CLEAR LOGFILE statement. After the log is cleared, the status changes to UNUSED.
CLEARING_CURRENT - The current log is being cleared of a closed thread. The log can stay in this status if there is some failure in the switch such as an I/O error writing the new log header.
INACTIVE - The log is no longer needed for instance recovery. It may be in use for media recovery. It might or might not be archived.
The checkpoint position will never lag the end of the log data by more than one log file since the longest the span can be is 90% of the smallest log。
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