RE: ** CPU impact of I/O change

Interesting...I always believed that row chaining occurred in the second case, when an update meant that the entire row would no longer fit. So are you saying that row chaining occurs on insert, and migration on update?  

Paul Baumgartel
CREDIT SUISSE
Information Technology
Prime Services Databases Americas
One Madison Avenue
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Phone 212.538.1143
paul.baumgartel_at_credit-suisse.com
www.credit-suisse.com  

From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Daniel Fink Sent: Tuesday, April 15, 2008 8:10 AM
To: ajoshi977_at_yahoo.com
Cc: oracle-l_at_freelists.org
Subject: Re: ** CPU impact of I/O change

The first question is "Why?"

Are you making these changes to address performance issues? If so, what is the impact of these chained rows? If you can't quantify the performance impact, you can't quantify the improvement.

The second question is "Will reorganizing tables fix the problem?". The answer to that is almost certainly not. Chained rows occur because the row is too large to fit into a block. Migrated rows occur when the update to a row needs more space than the available free space in the block. In the first case, either make the row smaller or the block (actually the available free space in the block) larger, two things that a truncate and reload won't help. In the second case, you have to change the application logic or the free space setting (pctfree) for the table. Migrated rows are a symptom of updates that cannot fit into the available free space in the block, something a truncate and reload won't help either.

Changing the HWM only impacts full table scans. If the HWM has been set too high because of a one time event (huge delete) and it has a quantifiable impact, then a reorganization of the table is appropriate. If the HWM is set too high because large insert/delete operations are normal, the change will be temporary.

Regards,
Daniel Fink

-- 
Daniel Fink

Oracle Performance, Diagnosis and Training

OptimalDBA    http://www.optimaldba.com
Oracle Blog   http://optimaldba.blogspot.com

A Joshi wrote: 

	Hi,
	   I plan to make changes to database like increase SGA, save
rows of some tables : then do truncate of table and insert the rows back
or move table to reduce HWM and to take care of chained rows. All this
will help reduce the I/O done. There are some full table scans and some
index accesses. My question is : What impact will this change in I/O
have on CPU usage. Improvement, no impact or adverse? Anything else  to
look at or watch out for? Thanks 
	
	
	

	

	


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