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RE: db_file_mutliblock_read_count and physical IO

From: Bobak, Mark <Mark.Bobak_at_il.proquest.com>
Date: Wed, 18 Aug 2004 11:11:55 -0400
Message-ID: <4C9B6FDA0B06FE4DAF5918BBF0AD82CF09EFEB18@bosmail00.bos.il.pqe>


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_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of ryan.gaffuri_at_comcast.net
Sent: Wednesday, August 18, 2004 10:25 AM To: oracle-l_at_freelists.org
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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Received on Wed Aug 18 2004 - 11:08:47 CDT

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