Re: 10g RAC and db_multi_block_read_count

From: Greg Rahn <>
Date: Wed, 5 Aug 2009 09:08:33 -0700
Message-ID: <>

Could you cite the reference that you mentioned about the MBRC=8 on RAC suggestion?

What is your workload? DSS? OLTP?

Given that not setting db_multi_block_read_count basically results in a costing of 8 w/o system stats but I/O size is db_multi_block_read_count blocks or in this case 1MB.

I look at it like this: is it more efficient to execute a larger number of smaller I/Os or a smaller number of larger I/Os when doing a FTS operation (the answer is the latter)? Then I would ask myself, does it even make any difference if it is RAC or not, and if so, why? I am struggling to think of any significant difference that RAC would introduce, so I would comment that it makes no difference and if the comments you found have no technical explanation, I would classify it as mis-information.

On Wed, Aug 5, 2009 at 8:28 AM, Crisler, Jon<> wrote:
> If I allow Oracle 10g to automatically set the parameter
> db_multi_block_read_count, the value is determined to be 128 (8k
> blocksize).  However, I ran across some comments that suggest in a RAC
> environment that this large size might be a bad choice, and it should be
> backed down to 8.  I cannot find any info in statspack / AWR that
> suggests a smaller number is better in RAC.
> Does anybody have further insight into this value in a RAC environment ?

Greg Rahn
Received on Wed Aug 05 2009 - 11:08:33 CDT

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