Adaptive Log File sync was introduced in 11.2. The parameter controlling this feature, _use_adaptive_log_file_sync, is set to false by default in 11.2.0.1 and 11.2.0.2.
In 11.2.0.3 the default is now true. When enabled, Oracle can switches between the 2 methods:
- Post/wait, traditional method for posting completion of writes to redo log
LGWR explicitly posts all processes waiting for the commit to complete.
The advantage of the post/wait method is that sessions should find out almost immediately when the redo has been flushed to disk.
- Polling, a new method where the foreground process checks if the LGWR has completed the write.
Foreground processes sleep and poll to see if the commit is complete.
The advantage of this new method is to free LGWR from having to inform many processes waiting on commit to complete thereby freeing high CPU usage by the LGWR.
when mode switches, it will write message to LGWR trace file.
*** 2012-10-02 08:15:47.049
kcrfw_update_adaptive_sync_mode: post->poll long#=316 sync#=1719 sync=247657
poll=12382 rw=6191 rw+=13890 ack=0 min_sleep=10023
But per my experience, “Polling” mode will cause higher average wait time of “log file sync” and is not a stable LGWR algorithm for heavy OLTP workload. It can be disabled by “alter system”.
Reference
Understanding LGWR, Log File Sync Waits and Commit Performance