Re: log file sync event

My 5 kopejka.

Don't forget about a kind of trap when interpreting wait time from V$SYSTEM_EVENT. Consider the difference of the two cases below. Case #1. You are driving a car alone and waited on a red traffic light for 3 minutes. You record your "wait time". The total wait time would be 3 mins.
Case #2. There are some other cars in front and behind of you. Each driver waits approximately 3 minutes (depending of earlier or later he/she joined the queueue). So what would be the "total wait time" and how it is related to the severity of the bottleneck?

So you should keep in mind, that in OLTP system with a lot of concurrent sessions it often happens that several sessions are waiting for LGWR simultaneously (so called group commit). Actually this is nice feature. However for this particular case the 'log file sync' time might be misleading and presenting things much worse than these really are. So I'd suggest you to compare 'log file sync' with 'log file parallel write' time which is recorded by LGWR. In the absence of "1/3 buffer full" writes those on request from DBWR you can better estimate this rough indicator of possible bottleneck.

Actually high 'log file sync' times should not care too much. As it was already told by other contributors, the response time for user processes is the only thing that matter. If the problem really exists you should know that without any statistics directly from your unhappy users/customers or your boss. In that case try affected user session tracing or use some other profiling tool.

HTH

• Andrey

Sandeep Dubey wrote:
> I am using 10.2 on linux for OLTP system. I am getting log file sync
> as top event. I can not change disk or combine transactions in one or
> some thing other as suggested in metalink.
>
> Does rman backup has any effect on this event as I see this event
> going up during backup time and performance goes down the drain.
>
> How can I reduce this wait event?
>
> Thanks
>
> Sandeep
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