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  Oracle Tips by Burleson

Oracle Data Buffer Monitoring with STATSPACK

There are two ways to use STATSPACK to compute the data buffer hit ratio. In Oracle8i and beyond, we may use the stats$buffer_pool_statistics table. For Oracle 8.0, the stats$sesstat table should be used.

NOTE: There is an important difference between stats$buffer_pool_statistics in Oracle 8.0 and Oracle8i. If STATSPACK was back-ported into Oracle 8.0, the stats$buffer_pool_statistics view does not give accurate data buffer hit ratios for the DEFAULT, KEEP, and RECYCLE pools. Instead, there is only one pool defined as FAKE VIEW. This uses the stats$sysstat table and should be used for Oracle 8.0:

See code depot for full scripts

-- ****************************************************************
-- Display BHR for Oracle8
--
-- Copyright (c) 2003 By Donald K. Burleson - All Rights reserved.
-- ****************************************************************

set pages 9999;

column logical_reads format 999,999,999
column phys_reads format 999,999,999
column phys_writes format 999,999,999
column "BUFFER HIT RATIO" format 999

select
. . .
from
perfstat.stats$sysstat a,
perfstat.stats$sysstat b,
perfstat.stats$sysstat c,
perfstat.stats$sysstat d,
perfstat.stats$sysstat e,
perfstat.stats$sysstat f,
perfstat.stats$sysstat g,
perfstat.stats$snapshot sn
where
. . .
d.statistic# = 41
;

The method below is used for Oracle 8.1 and beyond:

See code depot for full scripts

-- ****************************************************************
-- Display BHR for Oracle8i & beyond
--
-- Copyright (c) 2003 By Donald K. Burleson - All Rights reserved.
-- ****************************************************************

select
. . .
from
perfstat.stats$buffer_pool_statistics old,
perfstat.stats$buffer_pool_statistics new,
perfstat.stats$snapshot sn
where
(((new.consistent_gets-old.consistent_gets)+
(new.db_block_gets-old.db_block_gets))-
(new.physical_reads-old.physical_reads))
/
((new.consistent_gets-old.consistent_gets)+
(new.db_block_gets-old.db_block_gets)) < .90
. . .
;

A sample output from this script is shown below:

 yr. mo dy Hr BUFFER_POOL_NAME     BHR
------------- -------------------- -----
2001-12-12 15 DEFAULT                .92
2001-12-12 15 KEEP                   .99
2001-12-12 15 RECYCLE                .75
2001-12-12 16 DEFAULT                .94
2001-12-12 16 KEEP                   .99
2001-12-12 16 RECYCLE                 .65

This script provides us with the data buffer hit ratio for each of the buffer pools at one-hour intervals. It is important that the KEEP pool always has a 99-100 percent DBHR. If this is not the case, data blocks should be added to the KEEP pool to make it the same size as the sum of all object data blocks that are assigned to the KEEP pool.

To summarize, the DBA can control the data buffer hit ratio by adding blocks within the Oracle parameters. Oracle recommends that the DBHR not fall below 90 percent.

 

The above text is an excerpt from "Creating a Self Tuning Oracle Database", by Rampant TechPress.  It is only $9.95 and all scripts in this tips can be immediately downloaded.


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