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RE: log buffer space

From: K Gopalakrishnan <kaygopal_at_yahoo.com>
Date: Sun, 16 Mar 2003 05:28:36 -0800
Message-ID: <F001.0056B51C.20030316052836@fatcity.com>


Arup:

Sorry for the delay ;-)

I have not seen this is documented anywhere, other than 'Oracle Performance Tuning' OReilly Peter & Mark Gurry (page 304) where he claims the log writer writes when it is 2/3 full... Here is the Original Text.

<QUOTE>
Log Buffer

The log buffer contains the information showing the changes that have been made to database buffer blocks. When the log buffer reaches one-third full (two-thirds full in Oracle 7.3), a user performs a commit, or a write takes place to the database,......

</UNQUOTE>

I don't have any Oracle 7.3 database, (for that matter no database now as I composing this in Zurich Airport waiting for a connecting flight to Bombay..), So I may not be able to test that. But last time I verified was on an Oracle 8.1 database where the log file writes used to be in the order up to 2/3 full.

You can do a simple test to prove this point. You can use oradebug to trace the log writer process and do a CTAS of any big table (with a big log buffer) and you will be able to see the writes and number of blocks written in a single write.

I am surprised , this is not documented anywhere in the Oracle Documentation or any of the Oracle University course notes.

Best Regards,
K Gopalakrishnan

-----Original Message-----
Sent: Friday, March 14, 2003 11:04 AM
To: Multiple recipients of list ORACLE-L

Dennis,

The 1MB condition was in 8i as well, at least in 8.1.7, as I mentioned in my original post.

I was always under impresssion that the flush is triggered by the buffer being 1/3rd full; but KG mentioned it was 2/3rd, not 1/3rd and I was wondering where he got that information from and if it's documented. It true, that will certainly invalidate most of the what the fine manuals and Oracle Support analysts have said.

Any ideas, anybody?

Arup

> Add one more condition:
> New in Oracle 9i, it will write when 1 meg is reached, so the 1/3
> criteria is never reached if you use a big buffer.

>

> Dennis Williams
> DBA, 40%OCP, 100% DBA
> Lifetouch, Inc.
> dwilliams_at_lifetouch.com
>
>

> -----Original Message-----
> Sent: Friday, March 14, 2003 8:59 AM
> To: Multiple recipients of list ORACLE-L
>
>

> KG,
>

> Thanks for the response. Just for my curiosity, is it documented somewhere
> that the log buffer if flushed when 2/3rd full? From the Manual
> /server.920/a96524/c09procs.htm#3158 (please check the URL in OTN, the
first
> part is not complete), here is an excerpt
>

> <MANUAL>
> LGWR writes one contiguous portion of the buffer to disk. LGWR writes:
> a.. A commit record when a user process commits a transaction
> b.. Redo log buffers
> a.. Every three seconds
> b.. When the redo log buffer is one-third full
> c.. When a DBWn process writes modified buffers to disk, if necessary
>

> </MANUAL>

>
> At least this was the behavior I tested in 8.1.7; although I have not
tested
> in 9i. This has also been stated by several Oracle Support Analysts.
>

> Thanks.
>

> Arup Nanda
>

> ----- Original Message -----
> To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
> Sent: Friday, March 14, 2003 7:32 AM
>
>

> > Arup:
> >
> > <NO FLAMES>
> > The second condition is not quite true. It is 2/3 full in the current
> > versions.
> > </NO FLAMES>
> >
> > It is very easy to test with the event 10046^8.
> >
> > KG
> >
> >
> > --- Arup Nanda <orarup_at_hotmail.com> wrote:
> > > AK,
> > >
> > > If the log buffer is at least 4MB, then increasing it will not help,
> > > rather it may hurt. The log buffer is flushed when any of the the
> > > follwoing occur
> > > (i) 1 MB is filled up
> > > (2) 1/3rd is filled up
> > > (3) every 3 seconds
> > > (4) when a checkpoint occurs
> > > (5) when a commit occurs.
> > >
> > > Therefore, see if any of these could be the problem. It's easy to
> > > check #s 4 and 3.
> > >
> > > As Kirti suggested, the problem could be due to the redo logs being
> > > on a busy disk, or even a slow one.
> > >
> > > HTH.
> > >
> > > Arup
> > > ----- Original Message -----
> > > From: Deshpande, Kirti
> > > To: Multiple recipients of list ORACLE-L
> > > Sent: Thursday, March 13, 2003 8:13 PM
> > > Subject: RE: log buffer space
> > >
> > >
> > > Increasing log_buffer size is an option, if it is really small.
> > > I would also check if the redo logs are on a busy disk. If so, try
> > > moving those (or other busy data files on the same disk) to other
> > > not-so-busy disks.
> > >
> > > - Kirti
> >
> >
> > =====
> > Have a nice day !!
> > ------------------------------------------------------------
> > Best Regards,
> > K Gopalakrishnan,
> > Bangalore, INDIA.
> > --
> > Please see the official ORACLE-L FAQ: http://www.orafaq.net
> > --
> > Author: K Gopalakrishnan
> > INET: kaygopal_at_yahoo.com
> >
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> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Arup Nanda
> INET: orarup_at_hotmail.com
>

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> Author: DENNIS WILLIAMS
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--
Please see the official ORACLE-L FAQ: http://www.orafaq.net
--
Author: Arup Nanda
  INET: orarup_at_hotmail.com

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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: K Gopalakrishnan
  INET: kaygopal_at_yahoo.com

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Received on Sun Mar 16 2003 - 07:28:36 CST

Original text of this message

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