RE: Log file sync spike

The stats

    redo synch writes
    redo synch time
appear for a session that sends a "sync" call to the log writer.

The stats

    redo writes
    redo write time
appear for LGWR and record the number and duration of writes.

Over a period as long as fifteen minutes, it is possible for a change in timing (two jobs running concurrently instead of serially) to cause the redo synch times (and log file sync waits) to change dramatically while the redo write time doesn't change at all.

Regards

Jonathan Lewis
http://www.oracle.com/technology/community/oracle_ace/ace1.html#lewis

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> Subject: Log file sync spike
> Date: Fri, 9 Jun 2006 16:01:03 -0000
> From: "Laimutis Nedzinskas" <Laimutis.Nedzinskas_at_landsbanki.is>
>
> I am trying to understand what is causing "log file sync" 5-15 minutes
> duration spike in my db.
> What puzzles me is that I can not see spike in the other activities on
> my db.
>
> It is strange that:
>
> - "redo write time" statistics does not follow "redo synch time" at the
> time of the spike
> - v$sysstat value of "redo write time" is half of the "redo synch
> time".
>
> If "redo synch time" spike happens because of hw problems then why
> "redo write time" does not follow it?
> If it is not hw issue then what else LGWR can be doing 50% of time(see
> the difference between redo write time and redo synch time) ?
> Or am I hitting yet another V$-bug?
>
> Database statistics in centiseconds follows:
>
> 56,105,687 cs redo synch time from v$sysstat
> 54,796,921 cs log file sync from v$system_event
>
> 24,163,771 cs redo write time from v$sysstat
>
> 162,672 cs log file switch completion from v$system_event
>

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