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_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]
Sent: Tuesday, December 19, 2006 2:52 PM To: Allen, Brandon
Cc: oracle-l_at_freelists.org
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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