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RE: buffer cache

From: Cary Millsap <cary.millsap_at_hotsos.com>
Date: Thu, 08 May 2003 08:52:04 -0800
Message-ID: <F001.00593F06.20030508085204@fatcity.com>


High BCHR is not *necessarily* bad, as Zhu Chao says. But, a systemwide BCHR in excess of 95% or so is often an indication of one or more inefficiencies that can result in greatly improved performance if corrected:

For more information, see my "Why a 99% buffer cache hit ratio is *not* ok", and perhaps Jonathan will tell you how to access his "The database gets better but the metrics look worse" article that he presented at IOUG.

Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com

Upcoming events:
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-----Original Message-----
Sent: Thursday, May 08, 2003 3:32 AM
To: Multiple recipients of list ORACLE-L

Hi,stephane,

    Though BCHR tuning method is widely critiqued, high buffer cache hit ratio is not necessary bad either, right?

    Maybe high cache hit ration and v$bh with a lot of free/cr block means too big data buffer.

    Here is an example of too big data buffer:

15:29:44 SQL> select /*+rule*/ status,count(*) from v$bh group by status;

STATU COUNT(*)
----- ----------

cr         55120
free       54656
scur       17696
xcur       24549

4 rows selected.

Elapsed: 00:00:01.32

    While in another database with large sga,I think this buffer cache is not too big:  

15:27:05 perfstat_at_BIDDB.EACHNET.COM> select /*+rule*/ status,count(*) from v$bh group by status;

 STAT COUNT(*)

cr 11162

read 1

xcur 1037413

Regards
zhu chao
msn:chao_ping_at_163.com
www.cnoug.org(China Oracle User Group)
----- Original Message -----
To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Monday, May 05, 2003 7:11 PM

> High BCHR, V$BH ...
>
> >----- ------- Original Message ------- -----
> >From: Arvind Kumar <arvindk_at_sqlstarintl.com>
> >To: Multiple recipients of list ORACLE-L
> ><ORACLE-L_at_fatcity.com>
> >Sent: Mon, 05 May 2003 02:41:36
> >
> >Dear All,
> >
> > how can i check if my buffer cache is bigger
> >than necessary ?oracle db
> >version is 8.1.7.
> >
> >
> >Thanks
> >
> >
> >Arvind
> >
> >
> >--
> >Please see the official ORACLE-L FAQ:
> >http://www.orafaq.net
> >--
> >Author: Arvind Kumar
> > INET: arvindk_at_sqlstarintl.com
> >
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> Regards,
>
> Stephane Faroult
> Oriole
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> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Stephane Faroult
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: zhu chao
  INET: chao_ping_at_vip.163.com

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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Cary Millsap
  INET: cary.millsap_at_hotsos.com

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Received on Thu May 08 2003 - 11:52:04 CDT

Original text of this message

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