RE: high db hit ratio and a lot of waits on db sequential reads
- From: "Bobak, Mark" <Mark.Bobak@xxxxxxxxxxxxxxx>
- To: <genegurevich@xxxxxxxxxxxx>, "oracle-l" <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 16:17:02 -0400
Hi Gene,
My first guess, based on very little information, is that you have an
application that has lots of inefficient SQL. That is, the application's SQL
could be returning the results with a lot fewer logical I/Os. Since you're
doing so many logical I/Os, some of those are bound to turn into physical I/Os.
Those are causing the 'db file sequential read' waits. (I wonder, what's your
average wait time on a db file sequential read event? With 48GB on the server,
and with an 18GB buffer cache, I wonder if you're servicing a lot of those
physical reads from the filesystem cache? Assuming you haven't enabled DIO.)
So, what I'm saying is, you're doing tons of I/O, and most of it is logical,
but, you're doing so much, that even the (relatively) small amount that turns
into physical I/O is enough to dominate your response time profile with db file
sequential read events.
Cary Millsap says it much better than I can, so, you should definitely read:
"Why a 99%+ Database Buffer Cache Hit Ratio is Not OK"
Which is usually available from http://www.hotsos.com/ but unfortunately, is
not currently, cause they just suffered a nasty crash and are still piecing
things back together.
Hope that helps,
-Mark
--
Mark J. Bobak
Senior Database Administrator, System & Product Technologies
ProQuest
789 E. Eisenhower, Parkway, P.O. Box 1346
Ann Arbor MI 48106-1346
+1.734.997.4059 or +1.800.521.0600 x 4059
mark.bobak@xxxxxxxxxxxxxxx
www.proquest.com
www.csa.com
ProQuest...Start here.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of genegurevich@xxxxxxxxxxxx
Sent: Tuesday, September 25, 2007 2:57 PM
To: oracle-l
Subject: high db hit ratio and a lot of waits on db sequential reads
Hi all:
I am working on tuning an app running against oracle 10.2.0.3 We have 48G
on the server; my db_cache is 18G. When I
look at the awr reports, I see db hit ratio being over 99% and a lot of
waits for db sequential reads. Based on the SQL
there are a lot of table reads based on the primary keys so that kind of
waits is reasonable. But the question is
if the hit ratio is that high , if we read mostly for the cache, why do we
do that many reads. Is there an explanation for that?
thank you
Gene Gurevich
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