Wait Event: Log File Parallel Write

Description
The Log File Parallel Write wait event occurs when the Log Writer (LGWR) process is waiting to write the contents of the redo log buffer cache to the online log files on disk. This wait event typically indicates I/O contention related to the disk subsystem that holds the online redo log files.

Scenario:
High Transaction Volume: The platform is experiencing peak load, with numerous customers trying to secure their tickets at the same time.

SQL Query: When a customer finalizes their booking, it triggers a COMMIT operation to save the transaction.

INSERT INTO bookings (user_id, train_id, seat_number, amount) VALUES (123, 456, 'A1', 99.99);
COMMIT;

Log File Parallel Write Occurrence:

As multiple users complete their bookings, the LGWR process is frequently activated to write the redo entries to disk. During this peak period, LGWR may experience delays due to I/O contention on the disks holding the online redo logs, resulting in increased wait times for the Log File Parallel Write event.

Factors

1. High Transaction Rate: A surge in booking transactions generates significant redo data.
2. I/O Contention: If redo logs are on slow or overloaded disks, LGWR can experience delays.
3. Hot Backup Mode: Keeping tablespaces in HOT BACKUP mode longer than necessary increases redo generation.
4. Disk Configuration: Using RAID 5 for redo logs can introduce latency, although some improvements have been made with intelligent disk caches.
5. Disk Activity: Redo log members should be on disks with minimal I/O activity from other sources.
6. CPU or Memory Bottlenecks: If the system is CPU or memory bound, LGWR may take longer to complete I/O operations.

Solutions

1. Reduce Redo Generation: Optimize SQL statements to minimize the amount of redo produced.
2. Limit HOT BACKUP Duration: Ensure tablespaces are not left in HOT BACKUP mode longer than necessary to reduce excessive redo generation.
3. Use NOLOGGING Options: For bulk operations, consider NOLOGGING or UNRECOVERABLE options to lessen redo generation.
4. Utilize High-Speed Disks: Store redo log members on high-speed disks to improve performance.
5. Avoid RAID 5 for Redo Logs: If possible, avoid RAID 5 configurations for redo logs due to historical latency issues.
6. Separate Disk Locations: Ensure redo log members are on disks with little to no I/O activity from other sources.
7. Experiment with Disk Stripe Size: If using file systems like ASM or ZFS, adjusting the disk stripe size can enhance performance for LGWR I/O.
8. Check System Resources: Monitor CPU and memory usage to ensure they are not bottlenecking LGWR.
9. Adjust CPU_COUNT: On servers with high CPU_COUNT, consider adjusting LGWR settings to optimize performance, especially if CPU_COUNT exceeds 128

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