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Tuning the buffer cache hit ratio


A logical read occurs whenever a user requests data from the database. It occurs whenever the data is in the buffer cache or whether the user process must read it from disk. If the data must be read from disk then a physical read occurs. Oracle keeps track of logical and physical reads in the V$SYSSTAT table.

Use the following SQL statement to determine the values required for the hit radio calculation:

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

The cache-hit ratio can be calculated as follows:

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

If the cache-hit ratio goes below 90% then:

  • For Oracle 8 and earlier: increase the initialisation parameter DB_BLOCK_BUFFERS.
  • For Oracle 9 onwards: increate the initialisation parameter DB_CACHE_SIZE.

I am grateful to Kamil Jakubovic for provided me with a single statement for calculating the cache hit ratio:

select
    100*(1 - (v3.value / (v1.value + v2.value))) "Cache Hit Ratio [%]"
from
  v$sysstat v1, v$sysstat v2, v$sysstat v3
where
  v1.name = 'db block gets' and
  v2.name = 'consistent gets' and
  v3.name = 'physical reads'


These notes have been tested against Oracle 7.3.4, Oracle 8.0.5, Oracle 8.1.5 and Oracle 9.0.1 and may apply to other versions as well.

This page represents one page of many pages of notes I made whilst working with Oracle 7 through to 10i. I now no longer work with Oracle databases, but hope that these notes will continue to be of benefit to others.



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