Re: Cache Hit Ratio from system views

From: Brian Peasland <dba_at_nospam.peasland.net>
Date: Tue, 21 Aug 2007 14:34:48 -0500
Message-ID: <46cb3258$0$28879$88260bb3@free.teranews.com>

Steve Howard wrote:
> On Aug 21, 2:18 pm, DA Morgan <damor..._at_psoug.org> wrote:

>> Bob Jones wrote:
>>> High BCHR is always better than low - provided everything else being equal.
>> Nonsense. Sorry but this is total mythological nonsense. A high BCHR may
>> be an indicator of nothing more than you write really lousy code.
>> --
>> Daniel A. Morgan
>> University of Washington
>> damor..._at_x.washington.edu (replace x with u to respond)
>> Puget Sound Oracle Users Groupwww.psoug.org

>
> Correct, but he has consistently stated that "all other things
> equal...". In other words, let's assume that you have read every book
> by Jonathan Lewis, Tom Kyte, all the Oracle documentation (I know that
> is a sore spot for some people), ensured your hardware is properly
> configured, fill in whatever else you want...
>
> If your BCHR is 50% on one system and 99% on another (once again,
> identically coded, hardware, etc.)...
>
> is that not an indication that your DBA *may* have forgotten a couple
> of zeroes for db_cache_size parameter?
>
> I am truly curious, as this should be a yes or no answer. If the
> answer is anything other than no, then it does have *some* value.
>

I don't know that I could answer yes or no to that question. What do those numbers say about end user performance? Nothing. The DBA may have incorrectly sized the buffer cache...or it may be just right. The above numbers do not tell us which case is true.

By not being able to answer yes or no, haven't we said something about the usefulness of the BCHR? I think we have. We hae said that the BCHR is not a reliable indicator of anything useful.

Cheers,
Brian

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Brian Peasland
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Received on Tue Aug 21 2007 - 14:34:48 CDT