When a user issues a commit or rollback then the redo data in the redo buffer is written to the online redo log file. The user session waits for this event to finish before continuing with other processing. This wait time is represented as log file sync wait event.
A number of people have asked the question as what is the difference between log file parallel write and log file sync.
The difference is....
log file parallel write occurs when LGWR writes redo records from redo buffer to online redo log file. This may take place very frequently when it meets any one of the following condition,
1. Once in every three seconds.
2. _LOG_IO_SIZE threshold is met.
3. 1MB worth of redo entries are buffered.
4. Commit.
5. Rollback.
6. When DBWr requests.
The user sessions will never experience the log file parallel write wait event.
When the user session issue commit or rollback then it leads to log file sync wait event, which the user will experience by response time.
Thanks
Sunday, September 13, 2009
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1 comment:
Very good explanation. Thank you.
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