Oracle db_file_multiblock_read_count Don Burleson

10gr2 Note:  Starting in Oracle 10g release 2, Oracle recommends not setting the db_file_multiblock_read_count parameter, allowing Oracle to empirically determine the optimal setting.  For more details, see my notes on 10gR2 automatically tuned multi-block reads.

When you implement multiple blocksizes you should set your db_block_size based on the size of the tablespace where your large-object full-scans will be occurring. The Oracle 10g release 2 Performance Tuning Guide page 14.4 notes:

"In release 10.2, the optimizer uses the value of mbrc when performing full table scans (FTS). The value of db_file_multiblock_read_count is set to the maximum allowed by the operating system by default. However, the optimizer uses mbrc=8 for costing.

The "real" mbrc is actually somewhere in between since serial multiblock read requests are processed by the buffer cache and split in two or more requests if some blocks are already pinned in the buffer cache, or when the segment size is smaller than the read size.

The mbrc value gathered as part of workload statistics is thus useful for FTS estimation."

Remember, the parameter db_file_multiblock_read_count is only applicable for tables/indexes that are full scanned, but it also effects the SQL optimizer in its calculation of the cost of a full-table scan.

According to Oracle, this is the formula for setting db_file_multiblock_read_count:

                                 max I/O chunk size
db_file_multiblock_read_count  = -------------------
                                 db_block_size

But how do we know the value of the max I/O chunk size? 

The maximum effective setting for db_file_multiblock_read_count is OS and disk dependant. Steve Adams, an independent Oracle performance consultant (see www.ixora.com.au ), has published a helpful script to assist you in setting an appropriate level.  This script conducts a test and sample actual I/O chunk sizes on your server to aid you in setting db_file_multiblock_read_count:

-------------------------------------------------------------------------------
--
-- Script: multiblock_read_test.sql
-- Purpose: find largest actual multiblock read size
--
-- Copyright: (c) Ixora Pty Ltd
-- Author: Steve Adams
--
-- Description: This script prompts the user to enter the name of a table to
-- scan, and then does so with a large multiblock read count, and
-- with event 10046 enabled at level 8.
-- The trace file is then examined to find the largest multiblock
-- read actually performed.
--
-------------------------------------------------------------------------------
@save_sqlplus_settings

alter session set db_file_multiblock_read_count = 32768;
/
column value heading "Maximum possible multiblock read count"
select
value
from
sys.v_$parameter
where
name = 'db_file_multiblock_read_count'
/


prompt
@accept Table "Table to scan" SYS.SOURCE$
prompt Scanning ...
set termout off
alter session set events '10046 trace name context forever, level 8'
/
select /*+ full(t) noparallel(t) nocache(t) */ count(*) from &Table t
/
alter session set events '10046 trace name context off'
/

set termout on


@trace_file_name


prompt
prompt Maximum effective multiblock read count
prompt ----------------------------------------

host sed -n '/scattered/s/.*p3=//p' &Trace_Name | sort -n | tail -1

@restore_sqlplus_settings