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Re: Cache Hit Ratio from system views

From: Niall Litchfield <niall.litchfield_at_gmail.com>
Date: Wed, 08 Aug 2007 20:51:15 -0000
Message-ID: <1186606275.616321.166050@w3g2000hsg.googlegroups.com>


On Aug 8, 9:14 pm, DA Morgan <damor..._at_psoug.org> wrote:
> Niall Litchfield wrote:
> > sybra..._at_hccnet.nl wrote:
> >> On Wed, 08 Aug 2007 12:02:46 -0700, Teresa Masino
> >> <teresa.mas..._at_peninsula.org> wrote:
>
> >>> Alone, yea, but they can sometimes be an indicator that an instance
> >>> needs attention.
> >> Do you really think so? Yesterday I had a customer spouting flames
> >> because of a RAC database coming down to a crawl due to buffer busy
> >> waits in conjunction with both instances updating the same table over
> >> and over again.
> >> Nothing to be seen from the BCHR.
>
> >> But then of course BCHR is a MIPS (Meaningless Indicator of the
> >> Performance of the System).
> >> Only good old Don Burleson makes people erroneously believe they can
> >> tune their database by throwing memory at the problem to crank up the
> >> BCHR.
> >> Savy professionals (a favorite phrase of Don) know better.
>
> > enough of you will recognise where I stand on ratios. Indeed I believe
> > that I presented on the effectiveness or in fact otherwise of the BCHR
> > on its own nearly 5 years ago.
>
> > I have some sympathy withe Teresa's sentiment above though. Imagine a
> > system that has had a truly horrible burleson concerned hopelessness
> > ratio of 75% since 2002. Today the BCHR is 25% (or indeed 98%). I
> > venture to suggest that it's a change worth determining the reasons for,
> > either way. Ratios don't mean anything much, abrupt changes in them
> > often do.
>
> On the other hand monitoring things that matter would save you the
> trouble of chasing after smoke in a house of mirrors.
>
> There are real things that can be monitored. And if Theresa has the
> license to use AWR as it seems she'd be far better served looking at
> things that matter.
> --
> Daniel A. Morgan
> University of Washington
> damor..._at_x.washington.edu (replace x with u to respond)
> Puget Sound Oracle Users Groupwww.psoug.org- Hide quoted text -
>
> - Show quoted text -

Indeed. And since EM will do alerts based on user defined metrics, it's possible the whole thing is moot.

Niall Received on Wed Aug 08 2007 - 15:51:15 CDT

Original text of this message

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