Re: Read Hit Ratio Question- Suggestions Welcome.

Hi Tapan
Sometimes the Consistent Gets parameter gets so High (when the Instance is up and running for months together) that it skews up the Result..It reflects the accumulated value rather than the current scenario..

There was one more issue related to Consistent Gets that it becomes a negative number sometimes which by the way is base bug 251434, fixed in 7.2.

The Better way to determine the Buffer Cache hit Ratio is to run the UtlBstat/Estat reports during the Peak time and then determine the Read hit Ratio.

Also note that in 7.3.4 and Oracle 8 ,Changes were made to the 'physical Reads' parameter which now includes direct block reads as well as reads to get data into the buffer cache. Hence the above formula only gives an lower bound for the hit ratio on these releases.

Hope this Helps..

-Thiru

Tapan Trivedi wrote:

> Hey Gurus,
> I am doing this query to find out the Read Hit ratios on a Baseline
> system (upon which further development will be done) .
>
> select 1-(sum(decode(name, 'physical reads', value,0))/
> (sum(decode(name, 'db block gets', value,0)) +
> (sum(decode(name, 'consistent gets', value,0))))) * 100
> "Read Hit Ratio"
> from v$sysstat;
>
> I am getting very weird results on about four systems on which I have
> run the queries . I am getting NEGATIVE, yes, NEGATIVE results. What
> does this mean ?
>
> I am getting values like
> -.047
> -18.99
> -2.31
> -.69 etc.
>
> What does this mean ?
>
> Tapan
Received on Thu Aug 05 1999 - 11:48:57 CDT