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RE: IO wait

From: Binley Lim <>
Date: Tue, 23 Jul 2002 16:43:21 -0800
Message-ID: <>

Top is a I/O good indicator, but too broad. You need to start with the SQL, both logical and physical (sequential and scattered reads).

Always use histograms, this can do great things with minimal effort. With OLTP, there is parsing cost component you need to consider. With DW, this is a non-event compared with the savings you can gain from sensible CBO decisions. From the logic of the SQL, then you can assist the CBO by trying different partitioning schemes, range/hash combined, to enable pruning, or at least enable partition-wise joins.

Then you can look at all the file-system/physical volume I/O topics that people have written papers about. Sun's site is good place to start.

>>> DWILLIAMS_at_LIFETOUCH.COM 07/24/02 07:44a.m. >>>
Jack - Is your load process CPU-bound (high CPU utilization), or I/O-bound (high I/O statistics)? My guess is that from waits you want to confirm that I/O is your high wait statistic, to rule out any other problems that may be slowing down your load time. Then you may want to make a list of ideas that may speed your load and benchmark them (run the same load multiple times under the same circumstances) and see what change or combination of changes reduces the load time. Once you have a new approach that is yielding better load performance, you may want to check the wait times again to confirm that another problem isn't slowing your load.

Dennis Williams
Lifetouch, Inc.

-----Original Message-----
Sent: Tuesday, July 23, 2002 10:58 AM
To: Multiple recipients of list ORACLE-L


We are tuning a new vital process on our data warehouse, and it is IO intensive - lots of parallel direct reads and writes. During our testing we are driving IO wait to ~60% (per top).


  1. is top a valid measure of IO wait?
  2. Is a high io wait an issue to be concerned about?
  3. how else can it be accurately measured?
  4. How can I link IO wait to what is happening inside the database?



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Author: Binley Lim

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Received on Tue Jul 23 2002 - 19:43:21 CDT

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