log file parallel write Wait Event , Cause, Related SQL queries, Possible Resolution

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The "log file parallel write" wait event occurs when Oracle is writing redo log information to multiple redo log files in parallel. This wait event is related to the process of committing transactions and ensuring that redo log records are safely stored on disk. 

 Explanation: 

 During a transaction commit, the changes (redo log records) made in the transaction need to be written to the redo log files on disk to ensure data durability. 

When multiple redo log files are configured and the database is set up for parallel writing, the "log file parallel write" wait event occurs as Oracle writes redo log records to multiple log files simultaneously. 

 Queries: 

 Identifying Sessions with "log file parallel write" Wait Event:

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

Use the following queries to find high commit sessions:

 

select sid, value from v$sesstat
where statistic# = select statistic# from v$statname where name = 'user commits') order by 2 desc;

A high redo wastage also indicates high frequency commits

 

select b.name, a.value, round(sysdate - c.startup_time) days_old
from v$sysstat a, v$statname b, v$instance c
where a.statistic# = b.statistic#
and b.name in ('redo wastage','redo size');

Optimize Redo Log Configuration: Ensure that the number of redo log groups, members, and size are appropriately configured based on the workload. Having more redo log groups can help distribute parallel writes. 

 Monitor I/O Performance: Regularly monitor the I/O performance of the storage subsystem where redo log files are stored. Ensure that it can handle the write load efficiently. 

 Use Faster Storage: Utilize high-performance storage for redo log files to minimize the time taken for parallel writes. 

 Redo Log Group Placement: Distribute redo log groups across different storage devices to avoid I/O contention. 

 Buffer Cache Tuning: Adequate buffer cache size can reduce the frequency of "log file parallel write" waits by keeping frequently accessed data in memory. 

 Batch Commit Operations: If possible, batch smaller transactions together to reduce the frequency of parallel writes. 

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

 Review Workload: Evaluate the application's transaction volume and patterns to ensure that redo log configuration matches the workload. 

 Remember that the "log file parallel write" wait event is a normal part of database operations involving transaction commits. Properly configuring and monitoring redo logs, I/O performance, and the overall system is essential for minimizing the impact of this wait event.

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