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RE: LGWR's Writing Habits

From: Steve Adams <>
Date: Thu, 14 Dec 2000 06:11:13 +1000
Message-Id: <>

Hi Gaja,

Have another look at that MAX, and you'll see that it has the opposite effect to what you claim. That is, the 1/3 threshold is effective *below* that threshold, not *above* it.

@ Regards,
@ Steve Adams

-----Original Message-----
From: Gaja Krishna Vaidyanatha [] Sent: Thursday, 14 December 2000 3:45
To: Multiple recipients of list ORACLE-L Subject: LGWR's Writing Habits

Hello everyone,

A couple of days ago I had posted a response to someone's query about copy latches and in that discussion I had mentioned that the 1/3 full event of the redo log buffer will really not 'kick in' unless log_buffer is set to at least 1Mb. I must confess that the number here is actually 3Mb and not 1Mb as posted before. I apologize for that typo.

However, the fact remains that from Oracle8, LGWR will not be posted by the 1/3rd full event of the redo log buffer, unless there are at least 1 Mb worth of redo entries in the log_buffer. In my previous posting, I said that for this to occur the log_buffer should be sized at least 1 Mb, but that number is actually even higher - 3 Mb.

And the rationale behind that stems from the events on which LGWR writes to disk. They are as follows:

  1. Every 3 secs (independent of DBWR, yes this has been proven to be true by testing and looking up the source code).
  2. On a Commit.
  3. When DBWR posts LGWR to write. (This is to make sure that the redo entires are written before 'dirty' blocks are written to disk).
  4. On checkpoints.
  5. When the log_buffer is 1/3rd full, but subject to the following formula MAX(1 Mb., log_buffer/3), i.e., at least 1 Mb of redo entries need to be there, for the 1/3 event to occur.

Now let's think about this for a minute. Realistically the need for the 1/3rd full event is not that critical, as events 1-4 will take care of getting the redo entries down to disk. And this in no way will cause more 'log buffer space' waits than what the system is already experiencing.

To make doubly sure that I was not 'under the influence', I called a very good friend of mine at Oracle, and had him check the source code for 8.0.5, and he corroborated my claim. We are doing some further digging, to determine whether 'all' or 'one' of the redo copy latches are used to protect the buffer, while 'flushing' the entire contents of the log_buffer. There are 2 references to this (one mentioning 1 copy latch and the other mentioning all copy latches) and we will get to the bottom of that and I will then post our findings.

End of technical stuff;

Begin non-technical stuff;

When I originally posted that the 1/3rd full event has changed in functionality in Oracle8, it was termed as a 'ludicrous suggestion'. Now, that comment may have gone 'just a bit too far' to my personal comfort, as it was directed against me 'point blank'. I just could not hit the 'delete key' on that one.

In light of all the 'country of origin' and 'ethnic background' stuff that has gone through our list in the past couple of days, I humbly urge all of us to exercise some caution in our communications. This is an forum where we cannot guage the poster's feelings, hence we need to make sure that we are sensitive to the reader's feelings.

While it is absolutely OK to question the contents of a posting, please do it in a manner that is 'amicable'. After all, we are human beings first, aside of our ethnic, cultural, religious and political backgrounds. Let's try to respect thatm core fact.

When I met Jared approx. 3 years ago at an IOUG-A conference, we talked about creating an Oracle listserv to 'share relevant and accurate information'. I don't think Jared or any one of us, would want this to become 'electronic battleground' for food fights or to be a forum to 'force and assert one's technical or personal ego'.

This list today has a few thousand individuals from all corners of the world, and all of us contribute to the best of our abilities. Let's continue the great technical content exchange that we have had and let's do that with 'respect for the reader'. Thanks for your patience, tolerance and technical prowess.

End non-technical stuff;

Seasons Greetings,


Gaja Krishna Vaidyanatha
Director, Storage Management Products, Quest Software Inc. Office : (972)-304-1170, E-mail :

Author:Oracle Performance Tuning 101 by Osborne McGraw-Hill "Opinions and views expressed are my own and not of Quest"

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Author: Gaja Krishna Vaidyanatha

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Received on Wed Dec 13 2000 - 14:11:13 CST

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