Re: ** CPU impact of I/O change
- From: Daniel Fink <daniel.fink@xxxxxxxxxxxxxx>
- To: ajoshi977@xxxxxxxxx
- Date: Tue, 15 Apr 2008 06:10:20 -0600
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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