Re: Is DBFMRC limited to 16 on AIX ?

We're running Oracle 9i (9.2.0.7) and 10g (10.2.0.2) on AIX 5L (5.3 ML5) connected to DS4800 (yuck) SANs. When you evaluate the dbfmbrc, in addition to the point Mark has made, you need to consider any prefetch algorithm built into your storage, coupled with the size of your read cache, coupled with your storage bandwidth(TCP, internal, fibre), coupled with your storage (SAN?) element/block size, coupled with your OS filesystem block size.

We've got our dbfmbrc set to 8 with 128k element size on the SAN, Oracle block size of 8192, OS filesystem block buffer of 4k, fibre attached at 2GB (in my dreams), 4GB read cache with prefetch setting currently unknown to me.

We're using CIO.

On Mon, Aug 4, 2008 at 11:24 AM, Bobak, Mark <Mark.Bobak_at_proquest.com>wrote:

> Hemant,
>
> I'm not an AIX guy, but, what's your db_block_size? If it's 8k, that
> means w/ dbfmrc=16, you're doing 128k reads.
>
> Is there some reason AIX is limited to 128k reads?
>
> I know the limit on Solaris and Linux is 1M.
>
> -Mark
> ________________________________________
> From: oracle-l-bounce_at_freelists.org [oracle-l-bounce_at_freelists.org] On
> Behalf Of Hemant K Chitale [hkchital_at_singnet.com.sg]
> Sent: Monday, August 04, 2008 10:51 AM
> To: oracle-l_at_freelists.org
> Subject: Is DBFMRC limited to 16 on AIX ?
>
> Testing db_file_multiblock_read_count at 128 on AIX we found much
> poorer performance (higher read times of 35ms) then that for dbfmrc
> at 16 (3ms).
>
> One of the team (not from Oracle) referred us to this section
> (A.2.5) of the Oracle Database Administrator's Reference for Unix
>
> http://download.oracle.com/docs/cd/B19306_01/server.102/b15658/appa_aix.htm#sthref792
> "Setting a large value for the DB_FILE_MULTIBLOCK_READ_COUNT
> initialization parameter usually yields better I/O throughput on
> sequential scans. On AIX, this parameter ranges from 1 to 512, but
> using a value higher than 16 usually does not provide additional
> performance gain.".
> How did Oracle come up with this limit of 16 for AIX ?
> Is it only for for DIO / CIO or NOT for DIO / CIO ?
>
> What experiences do Oracle on AIX administrators have with dbfmrc ?
>
>
>
> Hemant K Chitale
>
> http://hemantoracledba.blogspot.com
>
> "A 'No' uttered from the deepest conviction is better than a 'Yes'
> merely uttered to please, or worse, to avoid trouble."
> Mohandas Gandhi Quotes
> : http://www.brainyquote.com/quotes/authors/m/mohandas_gandhi.html
>
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