RE: db_file_multiblock_read_count causing full scans to takelonger?

Well, almost.  Here's what I get on 10.2.0.2.0 on AIX 5.3:

SELECT LEAST(dcs/(ses*dbs), 1048576/dbs) "CALC", dfmbrc
FROM
(
        SELECT a.value "DCS", b.value "SES", c.value "DBS", d.value
"DFMBRC"
        FROM v$parameter a, v$parameter b, v$parameter c, v$parameter d
        WHERE a.name = 'db_cache_size'
                AND b.name = 'sessions'
                AND c.name = 'db_block_size'
                AND d.name = 'db_file_multiblock_read_count'
);

      CALC  DFMBRC
----------  -------
68.2666667  66

At some point, I'll find out *why* this comes up this way.

Sorry for coming in late on the thread...just catching up...

Rich

p.s.  I know I didn't need to do the subquery.  I did it that way for
clarity.  :)

-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] 
Sent: Tuesday, December 19, 2006 2:52 PM
To: Allen, Brandon
Cc: oracle-l@xxxxxxxxxxxxx
Subject: RE: db_file_multiblock_read_count causing full scans to
takelonger?

Brandon

> Am I missing something, or isn't it best to just leave this parameter
> unset (default) in 10.2, in which case Oracle will automatically
adjust
> it to the max io size of your OS? 

Mhmm... I don't like that for the simplest reason that the maximum is
not the best value for performance! As the OP noticed...

> Along with computing system stats on a typical workload so you have an

> accurate mbrc, you should get the best of both worlds - accurate
costing
> and optimized sequential scans, right?

In some situations, as I just wrote, no.

> I have it unset on my 10.2 AIX 5.3 system and it has defaulted to 128
> with an 8k block size (1MB).

Notice that there are some limits... According to my tests the value of
db_file_multiblock_read_count is calculated with the following formula: 

min( DbCacheSize/(Sessions*DbBlockSize) , 1048576/DbBlockSize )


Regards,
Chris
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