Re: DB Cache Hit Rate > 100%

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)

The user is using a monitoring tool: VERITAS Savant, and the 156% hit rate is showing on the screen.

Like you, I have never seen a hit ratio > 100%, but I have seen cases where there is data corruption in the V$ views in long-running instances (this was noted in MetaLink a while back). I wonder if this might be the case?

Fred Received on Wed Sep 01 2004 - 09:35:41 CDT