RE: db_file_multiblock_read_count 10g default values
- From: "Allen, Brandon" <Brandon.Allen@xxxxxxxxxxx>
- To: Christian Antognini <Christian.Antognini@xxxxxxxxxxxx>, "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 6 Nov 2009 09:57:04 -0700
Hi Chris, yes it's helpful - at least nice to get some confirmation of my
findings. You may have seen from one of my later posts, the formula I
eventually came up with was very close to yours:
db_file_multiblock_read_count=(db_cache_size/db_block_size/sessions)*.95
Notice the 5% reduction and I also found there appears to be a limit of .28% of
db_cache_size in addition to the limit of 1048576/db_block_size (which I think
is actually max_io_size/db_block_size, so it could vary on different
platforms), so I guess it could be shown more completely like this:
db_file_multiblock_read_count=min(db_cache_size/db_block_size/sessions*.95,max_io_size/db_block_size,.0028*db_cache_size)
This was with 10.2.0.4 running on Oracle Linux 5.2, so it may be a little
different from when you did your tests back in 2005.
I also agree this method of "auto-tuning" isn't sucha a good idea since if you
set your db_cache_size/sga_target too low or your sessions too high, then you
may be inadvertently throttling down your multiblock reads more than you
should. I'm thinking in my specific situation I'll gather system_stats to set
mbrc for optimization and then set db_file_multiblock_read_count=128 manually
instead.
Thanks,
Brandon
-----Original Message-----
From: Christian Antognini [mailto:Christian.Antognini@xxxxxxxxxxxx]
My conclusion, which was also published in my book (page
178), was the following:
dfmbrc = min(1048576/db_block_size,
db_cache_size/sessions/db_block_size)
BTW, the dependency between db_cache_size and dfmbrc is, IMO, really
bad...
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