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RE: missed Anjo's webcast..

From: <>
Date: Thu, 08 Aug 2002 13:19:18 -0800
Message-ID: <>

I started working with Oracle 6.0. Back then, the BHR was one of THE standard ways of looking at performance issues, and even all the Oracle books back then recommended it. Low BHR, increase db_block_buffers. This was what was followed. I admit, I did it too. Then came Oracle 7.3 and Oracle 8 where a DBA could look at wait events. And then the works of Gurus like Craig Shallahamer showed that this was not the right way. These works convey to me that a BHR of 99% does not mean a highly efficient database. Vice Versa, a BHR of 50% does not indicate a poorly performing database. Give me a database with a 45% BHR, and I can get it to 99% by running a few queries. Point well understood. Thats all I read into it. It does not mean in any way that I should now ignore PIO's and start tuning LIO's. It also does not mean that if the BHR is 99%, I have an LIO problem.

I must admit that I do sometimes tend to use the BHR, like Mladen. It can be an indicator that theres something wrong with the database. I have this database performing well, the users are happy, the BHR is mostly at 90%, and now it suddenly shoots down to 70%. Someone's sneaked in a query he should not.

What you infer from the BHR is what counts.

Moi 0.02$


                    Mladen"              To:     Multiple recipients of list ORACLE-L <>        
                    <MGogala_at_oxhp        cc:                                                                        
                    .com>                Subject:     RE: missed Anjo's webcast..                                   
                    Sent by:                                                                                        
                    August 08,                                                                                      
                    2002 04:31 PM                                                                                   
                    respond to                                                                                      

Well, I guess that I disagree. Buffer hit radio does matter as one of the performance indicators, but
certainly not the only one. Your and Mr. Milsap thesis is that LIO also is very expensive and its cost
is far from being negligible, so having gazillion of LIOs instead of 100 times smaller number of PIOs will
not make our system run faster. BHR alone cannot be used to judge to overall health of the system, but
thebn again, there is no such thing as the "overall health of the system". It's the users of the system who
will say whether the performance is satisfactory or not, and I'm usually tuning an application, not an
imaginary "overall system". Low cache hit ratio usually tells me that I do have a hog who is using lots
of PIOs. By my experience, it usually is a very good indicator that something is wrong, at least on an OLTP
 system. So, after all, I do find BHR a useful indicator, but by no means the only one or the most important
one. Event 10046, SQL_TRACE (level 1 of 10046), explain plan and v$session_event still are the tools
I need most, but I still do need BHR as an indicator.

Mladen Gogala
Oracle DBA
Phone: (203) 459-6855

 -----Original Message-----
 From: Anjo Kolk []  Sent: Thursday, August 08, 2002 1:05 PM  To: Multiple recipients of list ORACLE-L  Subject: Re: missed Anjo's webcast..

 Moi wrong ;-) Jeeh, human after all

 To summarize the webcast:
 db-block-buffers do mattter. Too many LIO do matter. Too many PIO do  matter. But Buffer Cache Hit ratio doesn't matter ....... End user  satisfaction does matter.

 I am always willing to clarify any points that I made, you just have to  ask me l ....



 I had this dream that I missed the webcast - which I did. However,  someone said it wasn't very interesting but the conversation of the people  (gurus) left over was very interesting as there was good solid evidence  that he was incorrect and db_block_buffers do matter. Kind of inline with  the discussion about redos yesterday and my indexing/partition issues -  hmmm.

 -----Original Message-----
 From: Mladen Gogala []  Sent: Thursday, August 08, 2002 1:38 AM  To: Multiple recipients of list ORACLE-L  Subject: Re: missed Anjo's webcast.., go to Events->webcasts...  On 2002.08.08 00:53 Madhusudhanan Sampath wrote:
> Are transcript documents available anywhere?
> Regards
> Madhusudhanan S

 Mladen Gogala

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Received on Thu Aug 08 2002 - 16:19:18 CDT

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