RE: Performance issue after creating higher block size tablespace

Did your query plan change?

Did you also rebuild the indexes after you moved the tables?

Gather statistics? Are they a big change from the last time you gathered
statistics?

Is the new tablespace in files that occupy comparable underlying volumes in
terms of I/Os supported per unit time?

Is your storage in some flavor of SAME, or were the tables being selected
from formerly on independently operating units of i/o (especially from the
insert target) and now you've lumped them all together?

 

What was your db_cache_size before?

Are you memory lean on the  machine and using filesystems? Have you robbed
the OS of file caching space by adding to the SGA size?

 

Those are all bits of a partial change analysis you might do, not that
you've stepped in it. If one or more of them is on target (measure, don't
guess) then you might have a shortcut out of your problem. Others might add
to the list.

 

Now if you had a time machine, I'd say get in it and measure things to
evaluate what (if any) performance benefit there was to be expected if you
could get i/o service time to zero by moving to 8K. Then, if that idealized
ceiling of possible benefit was significant, figure what the likely benefit
was if everything meshed in your favor with no side effects. Then, if that
still seemed worthwhile, plan and engineer the move so that you ruled out in
advance negative side effects. (And I'm wondering why not rebuild the whole
thing at 8K if the database block size was measured to predict an
advantage.)

 

So what to really do now? See where the time is going. One often useful bit
of information is routing the output of the select to dev/null and seeing
how long that takes. If the lion's share of your time is in the select, fix
that. Likewise, if you queue up the results of the select in a single table
and just select from there and insert into the destination, does that reveal
a bottleneck on the insert side?

 

Before you would move back, you would want to have some evidence that moving
back would eliminate some problem. Unless of course the urgency now is such
that just getting back where you were right away is more important than
minimizing the amount of work to reach better performance. Then you could
pretend you went through the time machine, figure out where your time is
going and attack the problem from that standpoint.

 

Regards,

 

mwf

 

  _____  

From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Saad Khan
Sent: Thursday, February 25, 2010 11:24 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: Performance issue after creating higher block size tablespace

 

Hi gurus.

I've a production database running on oracle10.2.0.3 at SUSE linux 10. The
default DB_BLOCK_SIZE for the database is 4K. 

There was a performance complain coming from the users and developers asked
me to look into that. They particularly complained about one stored
procedure that was taking too much time. Now when I looked into the stored
proc, I saw the insert statement in one particular table which is something
more than 4 million rows  while selecting from a bunch of other tables.

So what I did, I created a new tablespace with the db_block_size 8K and
moved all the tables that were used in that SP in the new tbs. 

And guess what, the new response came after that showed its taking almost
double the time as it was taking earlier. The AWR report shows a lot of user
IO activity and the tablespace that is hit most is the new one. Now is it
due to the different block size for this new tablespace? Is Oracle finding
it hard to manage 8k blocks inside the SGA designed for 4K originally?

The db_cache_size is set to 8192 and db_8k_cache_size is also set to 8192.

Is there any other step I can take? I dont want to revert it back to 4k , I
think it should work.

Any suggestions?

Thanks in advance.

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