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Re: Performance analysis (enqueue and buffer busy waits)

From: Jonathan Lewis <>
Date: Wed, 15 Aug 2001 03:24:42 -0700
Message-ID: <>

Ignore (or at least treat very lightly) the comments about DBWm.

Excessive database writes can cause log file sync waits, as dbwr calls lgwr to write the log protecting the blocks it is about to write. In this case, you will see v$session_event for the db writers showing log file waits. Unfortunately I have a mental block that makes me explain this phenomenon 100% the wrong way round every 6 months or so. And that is what I did last night.

I suspect you are also going to tell me that the anomalous difference between CPU and elapsed time on the one big update is because everything else is done by triggers, and the update is waiting for triggers to complete ;(

4GB of redo log in 50 minutes is quite a lot. Unless I've done the arithmetic wrong, that's close to 1.4MB per second (or 2.8 since you are using Oracle duplexing). It seems a little odd that you are getting 'log file sync' as a problem without getting (in your case) 'log buffer space' and 'log file ... write'.

What is your average log file write size ?
(redo blocks written / redo writes). and
what do you other 'redo%' stats look like over the period ?

How many CPUs ?

Jonathan Lewis

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Author: Jonathan Lewis

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Received on Wed Aug 15 2001 - 05:24:42 CDT

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