log file sync vs. log file parallel write ?

[Sybrand Bakker]

On 11 Dec 2002 14:48:41 -0800, [email]bchorng@yahoo.com[/email] (Bass Chorng) wrote:

>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.

The writes requested by a log file sync are evidently processed by
means of log file parallel writes. Any other explanantion doesn't seem
to make sense.
Log file sync are by design *synchronous*, the process requesting the
sync and the LGWR need to coordinate communications. This will make
waits for log file sync always longer.


Hth


Sybrand Bakker, Senior Oracle DBA

To reply remove -verwijderdit from my e-mail address

[Yong Huang]

[email]bchorng@yahoo.com[/email] (Bass Chorng) wrote in message news:<bd9a9a76.0212111448.64321ddc@posting.google. com>...

> 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 ) ?

It's hard to compare these two events because they're experienced by
different sessions: log file sync by user sessions, log file parallel
write (as well as log file single write) by background sessions
specifically LGWR. Total waits and time waited could be more or less
when you compare them (not always one is higher than the other).
Remember that when user sessions are not doing anything, LGWR could
still be busy writing, because log writing is not always triggered by
session commit. Also, don't forget the wait by LGWR writing logfile
headers, which is sequential.

Yong Huang