Key Facts About CLEAR UNARCHIVED LOGFILE

 

The command ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP <current_group>; can indeed be run while the database is open and running. 

There is no absolute need to stop the database and bring it to the MOUNT stage unless the database is already down due to the loss of the "CURRENT" redo log file. 

Let's clarify this step-by-step:

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Key Facts About CLEAR UNARCHIVED LOGFILE

1. Purpose of the Command:

   • It clears the contents of a redo log file that is no longer accessible or corrupted.
   • It is used to resolve situations where the redo log file is missing or damaged.

2. Running It Online:

• Yes, the CLEAR UNARCHIVED LOGFILE command can be issued while the database is open.
• However, if the redo log file is "CURRENT" (actively being written to), the database will likely crash before you can issue the command. In that case, you'll need to start the database in MOUNT mode to resolve the issue.

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When to Use MOUNT Mode

• If the Database Has Crashed:

  • If the database instance crashes because it cannot access the "CURRENT" redo log file, you must start the database in MOUNT mode to issue the ALTER DATABASE CLEAR UNARCHIVED LOGFILE command.

• Reason for MOUNT Mode:

• In MOUNT mode, Oracle does not open the database to users but allows administrative operations like clearing redo logs or performing recovery.

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Can You Avoid MOUNT Mode?

• If the Database is Open:

  • If the database is still running and the missing redo log is not the "CURRENT" one, you can run:

    ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP <current_group>;

    This command clears the redo log group and allows the database to continue functioning without interruption.

• For "CURRENT" Redo Log:

• If the lost redo log file is "CURRENT," Oracle cannot clear it directly while it is in use.
• You must let Oracle switch to another redo log group using:

  ALTER SYSTEM SWITCH LOGFILE;

  Once the "CURRENT" group is no longer active (status changes from "CURRENT" to "ACTIVE" or "INACTIVE"), you can clear it.

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Recommendations to Minimize Data Loss

1. Checkpoint Before Clearing:

   • Ensure a checkpoint is issued to flush all dirty buffers to disk:

     ALTER SYSTEM CHECKPOINT;

2. Archive Logs:

   • Always archive redo logs before clearing them to minimize data loss.

3. Monitor Alert Logs:

• Check the alert log for warnings or errors related to redo log operations.

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Best Practices for Recovery

1. If the Database is Running:

   • Issue a log switch:

     ALTER SYSTEM SWITCH LOGFILE;

   • Clear the problematic redo log group:

     ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP <current_group>;

2. If the Database is Down:

• Start the database in MOUNT mode:

  STARTUP MOUNT;

• Clear the redo log group:

  ALTER DATABASE CLEAR UNARCHIVED LOGFILE GROUP <current_group>;

• Open the database:

  ALTER DATABASE OPEN;

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Conclusion

• You don’t need to stop the database to clear an unarchived redo log file if the database is running and the problematic log file is not "CURRENT."
• If the "CURRENT" redo log is missing and the database has already crashed, you'll need to start the database in MOUNT mode to proceed.

This approach ensures minimal data loss and proper recovery. Let me know if you'd like additional details!