RE: db_file_multiblock_read_count and performance

  • From: "Post, Ethan" <Ethan.Post@xxxxxx>
  • To: <ryan_gaffuri@xxxxxxxxxxx>, <Oracle-L@xxxxxxxxxxxxx>
  • Date: Mon, 6 Dec 2004 13:55:54 -0600

The assumption you make is that the OS actually uses "1" in step 3 and
"max" in step 5, I would want to confirm that.  Perhaps if you set the
value to something less than is actually possible it defaults to max?  

From: ryan_gaffuri@xxxxxxxxxxx [mailto:ryan_gaffuri@xxxxxxxxxxx] 
Sent: Monday, December 06, 2004 1:51 PM
To: Post, Ethan; Oracle-L@xxxxxxxxxxxxx
Subject: RE: db_file_multiblock_read_count and performance

i know how the CBO works. I've read the article. Per Oracle docs, they
lead you to believe, that oracle will do Physicall IO more efficiently
if you set that parameter higher and your system can use it. This does
not appear to be true. Here is how the test cases work.
1. determine max db_file_multiblock_read_count
2. bounce the instance to flush all the cache.
3.query with a full table scan with the value set = 1
4. bounce the instance to flush all the buffers
5. query with a full table scan with the value set to the max for my
No difference. I know query plans change with this setting. However, the
documentation appears to be a bit misleading lead me to believe this
alone would improve response time other than just tinker with the the
CBO estimates. 


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