RE: ** CPU impact of I/O change
Date: Tue, 15 Apr 2008 09:04:26 -0400
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?
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[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Daniel Fink Sent: Tuesday, April 15, 2008 8:10 AM
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.
-- 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? ThanksReceived on Tue Apr 15 2008 - 08:04:26 CDT
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