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Hit/Miss Ratios

Here are some scripts related to Hit/Miss Ratios .

Buffer Hit Ratio

BUFFER HIT RATIO NOTES:

  • Consistent Gets - The number of accesses made to the block buffer to retrieve data in a consistent mode.
  • DB Blk Gets - The number of blocks accessed via single block gets (i.e. not through the consistent get mechanism).
  • Physical Reads - The cumulative number of blocks read from disk.

  • Logical reads are the sum of consistent gets and db block gets.
  • The db block gets statistic value is incremented when a block is read for update and when segment header blocks are accessed.
  • Hit Ratio should be > 80%, else increase DB_BLOCK_BUFFERS in init.ora 

    select 	sum(decode(NAME, 'consistent gets',VALUE, 0)) "Consistent Gets",
	sum(decode(NAME, 'db block gets',VALUE, 0)) "DB Block Gets",
	sum(decode(NAME, 'physical reads',VALUE, 0)) "Physical Reads",
	round((sum(decode(name, 'consistent gets',value, 0)) + 
	       sum(decode(name, 'db block gets',value, 0)) - 
	       sum(decode(name, 'physical reads',value, 0))) / 
	      (sum(decode(name, 'consistent gets',value, 0)) + 
	       sum(decode(name, 'db block gets',value, 0))) * 100,2) "Hit Ratio"
from   v$sysstat

    Data Dict Hit Ratio

    DATA DICTIONARY HIT RATIO NOTES:

  • Gets - Total number of requests for information on the data object.
  • Cache Misses - Number of data requests resulting in cache misses

  • Hit Ratio should be > 90%, else increase SHARED_POOL_SIZE in init.ora 

    select 	sum(GETS),
	sum(GETMISSES),
	round((1 - (sum(GETMISSES) / sum(GETS))) * 100,2)
from 	v$rowcache

    SQL Cache Hit Ratio

    SQL CACHE HIT RATIO NOTES:

  • Pins - The number of times a pin was requested for objects of this namespace.
  • Reloads - Any pin of an object that is not the first pin performed since the object handle was created, and which requires loading the object from disk.

  • Hit Ratio should be > 85% 

    select 	sum(PINS) Pins,
	sum(RELOADS) Reloads,
	round((sum(PINS) - sum(RELOADS)) / sum(PINS) * 100,2) Hit_Ratio
from 	v$librarycache

    Library Cache Miss Ratio

    LIBRARY CACHE MISS RATIO NOTES:

  • Executions - The number of times a pin was requested for objects of this namespace.
  • Cache Misses - Any pin of an object that is not the first pin performed since the object handle was created, and which requires loading the object from disk.

  • Hit Ratio should be < 1%, else increase SHARED_POOL_SIZE in init.ora 

    select 	sum(PINS) Executions,
	sum(RELOADS) cache_misses,
	sum(RELOADS) / sum(PINS) miss_ratio
from 	v$librarycache