log file sync Wait Event , Cause, Related SQL queries, Possible Resolution

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The "log file sync" wait event occurs when a session is waiting for a commit's redo records to be written from the redo log buffer to the redo log file on disk. This wait event is crucial for ensuring data durability and is typically associated with transaction commits. 

 Explanation: 

 When a user or application commits a transaction, the changes (redo records) are first written to the redo log buffer in memory. 

The "log file sync" wait event occurs when the session waits for the database to confirm that the redo records have been successfully written to the redo log file on disk before acknowledging the transaction commit.

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Queries: Identifying Sessions with "log file sync" Wait Event:

SELECT SID, SERIAL#, EVENT, WAIT_TIME, SECONDS_IN_WAIT
FROM V$SESSION
WHERE EVENT = 'log file sync';

SELECT EVENT, TOTAL_WAITS, TIME_WAITED_MICRO/1000000 AS SECONDS_WAITED
FROM V$SYSTEM_EVENT
ORDER BY TOTAL_WAITS DESC;

Resolution: 

 Commit Less Frequently: If possible, batch commits to reduce the frequency of "log file sync" waits. However, balance this with the need for data consistency and durability. 

 Transaction Size: Larger transactions can help reduce the number of "log file sync" waits. Avoid very small transactions as they can increase commit overhead. 

 Tune Redo Log Size: Properly size redo logs to handle the workload. Small redo logs can cause frequent "log file sync" waits. 

 Redo Log Groups and Members: Have multiple redo log groups with multiple members to improve write concurrency. 

 Redo Log Placement: Place redo log files on separate disks to minimize I/O contention. 

 Fast Disk I/O: Use fast storage for redo log files to minimize the time taken to write redo records. 

 

Asynchronous Commit: Some systems support asynchronous commit, which reduces "log file sync" waits. However, this comes with a trade-off in terms of data consistency. 

Reduce Redo Generation: Optimize queries and transactions to generate less redo, reducing the need for frequent log writes. 

 Monitor I/O Latency: Monitor the I/O latency and ensure that disk subsystems can handle the write load. 

 Monitor and Analyze: Continuously monitor the database performance using tools like Oracle Enterprise Manager or other monitoring solutions to identify and address performance issues. 

 Remember that while minimizing "log file sync" waits is important, ensuring data consistency and durability is paramount. Tune your database and applications to strike the right balance between performance and data integrity.

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