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From: John Kanagaraj <>
Date: Thu, 16 May 2002 13:39:09 -0800
Message-ID: <>


> > Can I assume a i/o bottleneck from the following
> >
> > select * from v$system_event
> > order by TIME_WAITED;
> No. Wait events may only make up a small amount of
> processing that Oracle
> is doing for you.

Hmm.... I wouldn't think so. If there were just _one_ overall view that I could check to determine an Oracle bottleneck, it would be this view. A rollup of V$SESSION_WAIT is also effective in determining the _current_ bottleneck.

As to the original question: (And as with many other questions) It depends! The average_wait times displayed by V$SYSTEM_EVENT against specific events is a good indication of the 'bottleneck'. Just yesterday, I was debugging 'slow response' from a test database. The top wait events were for 'direct path read' and 'direct path write', with inordinate values for AV_WAIT (upto 932 CS or 9 secs!). This clearly pointed to some misconfigured Async I/O (It was on a Sun box where async_io default to TRUE). I then had the SA look at the Async config and it turns out that the Veritas layer was misconfigured - hopefully this has been sorted out. There have been numerous other examples - and I am not alone here - where DBAs have used V$SYSTEM_EVENT to determine the bottleneck. If I were 'S B', I would look at the avarage wait values for I/O and compare them to the manufacturer's claims. If they don't match or or not close, then it _may_ be an I/O bottleneck.

I would recommend purchasing (or even stoop to the level of purloining a copy from a friend!) the most excellent 'Oracle Performance Tuning 101' book written by Gaja and Kirti from this list. This question is dealt in great detail therein, and I am sure they would be happy to answer any further queries.

John Kanagaraj
Oracle Applications DBA
DBSoft Inc
(W): 408-970-7002

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Author: John Kanagaraj

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Received on Thu May 16 2002 - 16:39:09 CDT

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