log file sync vs. log file parallel write ?

Here is what Oracle says about these 2 wait events.

Log file sync:

When a user session commits (or rollback), the session redo information needs to be flushed to the redo logfile. The user session will post the LGWR to write the log buffer to the redo log file. When the LGWR has finished, it will post the user session...

SO this is very clear if you do not look at log file parallel write.

Now look at this.

Log file parallel write:

Writing redo records to the redo log files from the log buffer.

I really can't tell the difference from the above statements. But look at my v$session_event of a session, these 2 are way different:

 EVENT                                        TOTAL_WAITS TIME_WAITED
-----------------------------------------------------------------------
log file parallel write                         1576423     20189
log file sync                                   1779889    337294

So is log file parallel write part of log file sync ? If so, what's the difference called ? How can it be so big ( from the above statements, the difference would be the time writing to redo log buffer which should be very fast ) ?

Thanks for sharing your knowledge. Received on Wed Dec 11 2002 - 16:48:41 CST