How to calculate buffer cache hit ratio
February 16th, 2006 By Vadim Bobrov
There is no universal formula. The one most commonly used:
1 - ( physical reads / ( consistent gets + db block gets ) )
A better formula is:
1 - ( ( physical reads - (physical reads direct + physical reads direct (lob)) ) /
( db block gets + consistent gets - (physical reads direct + physical reads direct (lob)) )
In Oracle8.0 onwards it is possible to use multiple buffer pools. The hit ratios for each pool can be calculated using:
SELECT
name,
1-(physical_reads / (consistent_gets + db_block_gets ) ) "HIT_RATIO"
FROM V$BUFFER_POOL_STATISTICS
WHERE ( consistent_gets + db_block_gets ) !=0
except for 8.1.7 where due to a bug one must resort to the previous formula.
Vadim Bobrov
Oracle Database Tools
http://www.fourthelephant.com