Re: DB Cache Hit Rate > 100%

Fred <noway_at_jose.com> wrote in message news:<noway-CFC9A5.10354101092004_at_news101.his.com>...
> In article <4135cda9$0$5288$afc38c87_at_news.optusnet.com.au>,
> "Howard J. Rogers" <hjr_at_dizwell.com> wrote:
>
> > Fred wrote:
> >
> > > It's early morning and I haven't had my usual dose of caffeine.
> > >
> > > A client is reporting their DB buffer cache is experiencing a hit rate
> > > of 156%. Normally, I wouldn't be alarmed by this bit of spurious data,
> > > but I would like to be able to explain to the customer how this might
> > > occur. Anyone?
> > >
> > > Fred
> >
> > What formula are they using to calculate that?
> >
> > It's a bit meaningless otherwise.
> >
> > I've never seen a hit ratio greater than 100% except when I attempt to
> > calculate it using mental arithmetic alone.
> >
> > Regards
> > HJR
>
> As best I can tell, the formula is simply:
>
> 1 - physical_reads / (consistent_gets + db_block_gets)

Well, maybe the data buffers and sort sizes and log buffers are just way too small, so consistent_gets is near zero and db_block_gets is way less than physical reads? The SGA is just getting so thrashed it has to re-read things unnecessarily in order to make buffers consistent? And the display won't display negative %?

Just some wild-ass thoughts on how to push it over 100.

jg

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