db_file_multiblock_read_count 10g default values

From: Allen, Brandon <Brandon.Allen_at_OneNeck.com>
Date: Thu, 5 Nov 2009 10:57:48 -0700
Message-ID: <64BAF54438380142A0BF94A23224A31E112E739C2A_at_ONEWS06.oneneck.corp>

Hi list,

I've got several Oracle database instances running on the same Oracle Linux 5.2 box and from the documentation and everything else I could find, it looks like db_file_multiblock_read_count (dbfmbrc) is supposed to default to the maximum IO size of the OS (in units of db_block_size), but I've got varying sizes from 24 to 82 for dbfmbrc even though they're all on the same host and have the same db_block_size (8k). Any idea what could cause the differences? I've also considered the db_cache_size since that does vary from database to database, but even among the databases with the exact same sga_target (400MB) and db_cache_size (148M), there is still significant variation in dbfmbrc from 24 to 36.

I'm aware of the auto-tuning features for this parameter in 10g, and I've checked the hidden parameters and found that all the databases have the same value of 8 for _db_file_optimizer_read_count, so I'm not concerned about these differences in dbfmbrc affecting explain plans - this is just more of a curiosity than a real problem at this point. I've also verified that all instances have v$parameter.isdefault='TRUE' for db_file_multiblock_read_count to make sure it wasn't adjusted manually.

I'm considering gathering system stats in these databases (they've never been gathered before) to set mbrc more accurately for the optimizer and then setting db_file_multiblock_read_count to 128 (1MB) manually. Anyone see a problem with that approach?


P.S. Yes, these are the same databases I posted about recently where I'm also getting sreadtim & mreadtim < 1ms

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Received on Thu Nov 05 2009 - 11:57:48 CST

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