RE: db_file_mutliblock_read_count and physical IO
- From: "Bobak, Mark" <Mark.Bobak@xxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 18 Aug 2004 11:11:55 -0400
Ryan,
The problem is that the statistic you're looking at (physical reads) =
does not mean what you think it means. Look at the definition of the =
'physical reads' statistic in the Oracle Reference manual (under =
Statistics Descriptions). It is:
"Total number of data blocks read from disk. This number equals the =
value of "physical reads direct" plus all reads into buffer cache."
So, it's the total number of data blocks read from disk. It's NOT the =
number of db file scattered read (i.e. readv()) calls. So, by varying =
db_file_multiblock_read_count, you'll vary the number of readv() calls =
and the number of blocks read per call, but you will NOT affect the =
total number of blocks read. Note that reducing the number of readv() =
calls reduces the number of context switches and is a good thing.
Hope that helps,
-Mark
PS For more info on read() and readv(), and setting =
db_file_multiblock_read_count, go to http://www.hotsos.com and check out =
"Why are Oracle's read events named backwards?" and "Predicting =
multi-block read call sizes", both by Jeff Holt.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of
ryan.gaffuri@xxxxxxxxxxx
Sent: Wednesday, August 18, 2004 10:25 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: db_file_mutliblock_read_count and physical IO
I did a 10046 trace and verified that I can get up to 128 blocks/IO with =
db_file_multiblock_read_count.
How do I metric this? I look at my total physical IOs when I did a =
tkprof report and my total number of physical IOs remained the same when =
I had the value set to 8 as when I had it set to 128?
Before I did this test, my assumption was:
'Oracle would do less total Physical IOs since I am retrieving more =
blocks per IO.'
That assumption proved false. Can someone explain why?=20
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