Hi,
Database buffer hit ratio shows ur database buffer cache hit.Hit means that whenever an application requests data and that data is in database buffer cache,Oracle doesnot have to perform an I/O to get the data from disk to the database buffer.
Higher the database buffer cache ,better the performance in terms of I/O.A good buffer cache hit ratio should be more than 90%.It should be in the range of 90-98.
U have to see the database buffer cache hit ratio during normal business hours and peak time and ove a period of time.If ur database buffer cache hit ratio is less than 90%,shutdown the database change the init.ora file and increase the DB_BLOCK_BUFFERS parameter and bounce back the database.Check whether the hit ratio has increased.
The following formula is used to get the cache hit ratio

select (1-(phy.value/(db.value+cons.value)))*100 "Cache Hit Percentage"
from v$sysstat phy,v$sysstat db,v$sysstat cons
where phy.name='physical reads' and
db.name='db block gets' and
cons.name='consistent gets';

****cache hit ratio should be more than 90%(Oracle
recommendation)

Check the cache hit ratio over a period of time and during peak business hours

Regards,
Rohit Nirkhe,Oracle DBA,OCP 8i
[email protected]