Tuning the database cache hit ratio

You need to tune the database cache hit ratio if:

  • the gethitratio on the library cache is > 90%
  • the pinhitratio on the library cache is < 1%
  • the get hit ratio on the dictionary cache is < 15%

Use this simple script to measure the cache hit ratio:

prompt Measuring the Cache Hit Ratio
prompt Increasing the DB_BLOCK_BUFFER 
prompt if cache hit ratio < 90%
prompt ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
select 1-(phy.value / (cur.value + con.value)) "Cache Hit Ratio",
round((1-(phy.value / (cur.value + con.value)))*100,2)  "% Ratio"
from v$sysstat cur, v$sysstat con, v$sysstat phy
where cur.name = 'db block gets' and
      con.name = 'consistent gets' and
      phy.name = 'physical reads' 

Reader Feedback

Tiercelin F. writes: I am no way experienced in Oracle Tuning, but your tip appears quite strange to me. "You need to tune the database cache hit ratio if: the gethitratio on the library cache is > 90%" I would rather say that you need to tune if gethitratio is < 90 % (your script seems to believe it too). As a Sybase ASE tuning engineer, I usually tune the data caches for sensitive data to more than 95%.

Rohit D. writes: There are number of books with much more detail on this subject. However, there should be three separate queries for three separate areas of SGA, namely:

1. For Dictionary / Rowcache hit ratio: (Ideal recommended >

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select sum(gets), sum(getmisses), (1 - (sum(getmisses) / (sum(gets) ))) * 100 Row_Hit_Ratio
from v$rowcache;

2. For Library cache hit ratio: (Ideal recommended > 99%)

select sum(pins) as hits,
 sum(reloads) as misses,
 round((sum(pins-reloads)/sum(pins))*100,2) as Lib_Hit_Ratio
from v$librarycache;

3. For buffer cache hit ratio:

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 Buf_Hit_Ratio
from v$sysstat;

The only other part of SGA is redo log buffer, which doesn't have any hit ratios to be calculated. It appears that the tip refers only to the last one.

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This was first published in May 2002

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