Re: db_file_multiblock_read_count and 10g
- From: Nuno Souto <dbvision@xxxxxxxxxxxx>
- Date: Sun, 25 Jan 2009 11:18:47 +1100
조동욱 wrote,on my timestamp of 25/01/2009 1:44 AM:
This is called autotuned MBRC - literally, Oracle autotunes the value of
MBRC.
I don't think this is the case in 10g. Maybe in 11, but not 10gr2.
See this:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14237/initparams047.htm#CHDFAFHE
There is a world of difference between a "default value" and "auto-tune".
Nowhere in that doco is it said it is auto-tuned. I do recall reading something
from Jonathan about it being so in 11g but I won't be using 11g anytime soon so
I can't say for sure.
The best practice is
* Set system stat's MRBC to your current
db_file_multiblock_read_count value, like
dbms_stats.set_system_stats('mbrc', 64). This would gurantee that
your execution plans would not be changed unexpectedly by changed
MBRC value.
* Then use autotuned MBRC
Another thing: if one has the db files in a SAN using RAID10, then perhaps
it's a good idea to bypass all this auto-magical auto-tuned lowest common
denominator stuff and set dbfmrc to a value close to the stripe size.
That is usually, but not always, 64KB. I don't know about you but I still have
to see Oracle magically figure out what the stripe size is in a SAN all the way
back from the OS...
--
Cheers
Nuno Souto
in sunny Sydney, Australia
dbvision@xxxxxxxxxxxx
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