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$sysstat phy where cur.name = 'db block gets' and con.name = 'consistent gets' and phy.name = 'physical reads' /
Tiercelin F. writes: I am no way experienced in Oracle Tuning, but your tip appears quite strange to me. "You need to tune the database cache hit ratio if: the gethitratio on the library cache is > 90%" I would rather say that you need to tune if gethitratio is < 90 % (your script seems to believe it too). As a Sybase ASE tuning engineer, I usually tune the data caches for sensitive data to more than 95%.
Rohit D. writes: There are number of books with much more detail on this subject. However, there should be three separate queries for three separate areas of SGA, namely:
1. For Dictionary / Rowcache hit ratio: (Ideal recommended >
select sum(gets), sum(getmisses), (1 - (sum(getmisses) / (sum(gets) ))) * 100 Row_Hit_Ratio from v$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