Re: testing memory parameters

From: DA Morgan <>
Date: Mon, 17 Mar 2008 08:26:46 -0700
Message-ID: <>

Serge Rielau wrote:
> DA Morgan wrote:

>> Ana C. Dent wrote:
>>> "" <> wrote in news:62772aa8-fdbd-42b7-
>>>> I think that I should consider buffer and library cache hit ratios.
>>>> Is this a good way?
>>> BUFFER CACHE HIT RATIO ia a mythical indicator of performance.
>>> It should be ignored because it is meaningless.
>> Just to reinforce what Ana wrote ... hit ratios are meaningless
>> nonsense promoted by people who still think they can apply Oracle
>> 7 bad practices to current versions of Oracle.
>> If you have one of those books that recommends such nonsense do
>> something useful with it. I suggest a recycling bin.

> Hmmm.. OK I bite. The only thing worse than admitting ignorance being
> remaining ignorant....
> While I agree that a high buffer pool hit ratio is not a guaranty for a
> well tuned system, in OLTP the lack thereof tends to be a guaranty for a
> badly tuned system (oftentimes caused by a lack of a good access path
> such as indexing).
> Thus a good hit ratio is generally a necessary, but not sufficient
> indicator for a well tunes application.
> I thought this was a universal rule for the major DBMS.
> Is my reasoning wrong as far as Oracle is concerned? If so where?
> Cheers
> Serge

The issue, as always, comes down to the fact that the ratio has no meaning. Lets say the ration is 96.7%. Is that good? Is it bad? Does it mean the end users are getting good response times?

None of the above.

If the ratio goes up does that mean things are better? If it goes down does that mean things are worse? Which things?

In a well instrumented environment such as Oracle there are many places you can go to find out information that will point to specific tuning targets. BHCR is not one of them.

The links provided by Ana C. Dent are the place to start.

Daniel A. Morgan
Oracle Ace Director & Instructor
University of Washington (replace x with u to respond)
Puget Sound Oracle Users Group
Received on Mon Mar 17 2008 - 10:26:46 CDT

Original text of this message