buffer cache hit ratio

[uadmin]

Hi

I am rather new to this so appologies in advance.

What i need to now is how i calculate the buffer cache hit ratio with the following calculation.

Hit ratio = 1 - (physical reads / (db block gets + consistent gets))

the sql i ran is as followed :

Select name, value From v$sysstat
Where name in ('db block gets', 'consistent gets', 'physical reads');

O/P
NAME VALUE
--------------------------------------- ----------
db block gets 245246
consistent gets 13754331
physical reads 3426485

i tried but can not see how when i run a sql script that gives me the answer i get 76% hit ratio


Can any one explain the calculation to me

Regards
Simon

Simon Peter Wickham
Email: s.wickham@zoom.co.uk

 

[dbtoo2001]

select round((1-(3426485 / (245246+13754331 )))*100)||'%' AS BUFFER_CACHE from dual;

BUFFER_CACHE
------------
76%


It is the number of times you have to access disk divided by the number of times you have read memory multiplied by 100 (to yield percentage).