Comments on: Database Writer Parameters – What is Wrong with this Quote?

By: Charles Hooper

Charles Hooper — Sun, 20 Jun 2010 17:37:26 +0000

It is hard to disagree with Kevin’s comment. However, just because the meaning of the _SMALL_TABLE_THRESHOLD hidden parameter (as discussed in another recent blog article) changed between Oracle release versions, does not necessarily mean that the meaning of these parameters will change. For example, if we check Oracle Database 11.1.0.7, we find the following:
_db_writer_max_writes: Max number of outstanding DB Writer IOs

_db_block_write_batch: Not a valid parameter on 11.1.0.7

_db_writer_max_scan_pct: Not a valid parameter on 11.1.0.7

_db_writer_max_scan_cnt: Not a valid parameter on 11.1.0.7

Someone might be wondering right now if it is safe to change the value of the last three of the four hidden parameters. So, maybe there *is* something wrong with this quote.

By: Noons

Noons — Sun, 20 Jun 2010 04:45:07 +0000

I’m with Kevin here: 3 hidden parameters, no one knows for sure what they do nowadays, we start micro-tuning to this extent, what happens when a real performance problem surfaces?
Coming from an wengineering background, I’ve always applied an 80-20 rule to tuning. Diminishing returns becomes a primary concern beyond that.
Sure: academically, it might be interesting to research what-if scenarios at this level of detail. Exactly how many employers have I found interested in paying for academics? Their number approaches the real meaning of NULL….

By: Charles Hooper

Charles Hooper — Fri, 18 Jun 2010 21:14:39 +0000

My concerns include the following (this is why I paused at this section of the book):
* I really do not know what those four parameters change. I suspect that the author is correct regarding their description, but I would probably want a second opinion for normal parameter changes, and at least three or four other sources recommending the same changes before I would consider touching a hidden parameter. Other than a very brief description found through an Oracle query (see http://hoopercharles.wordpress.com/2010/02/23/retrieving-the-hidden-oracle-parameters-and-saving-the-parameter-values-in-excel/) there really is not a lot of information to be easily found in printed form for these parameters (I have not checked for these parameters, but that seems to be the general rule).
* What is the long term effect of these parameter changes? If we modify the parameters and allow the write list to grow longer, what happens when a lot of dirty blocks in the buffer cache must be flushed to make room for consistent read versions of blocks (with undo applied) during intense activity?
* The answer of how much of a value increase is not specified – is there some sort of formula we can use to find exactly the right value? And if there is a right value, why is it not the Oracle Database default value?
* Where is the warning that changing hidden parameters should only be performed under the guidance of Oracle support? It was mentioned on one page (I cannot recall if the book stated to contact Oracle support), possibly in a footnote.
* Is there another way to improve performance without adjusting the hidden parameters – or is that the only way. Does Exadata use the modified parameters, if not why?
* Does this recommendation only apply to databases in the petabyte range, or is there some lower limit to database size (and rate of change) where the advice does not apply?
* What if a spfile is used rather than a pfile (init.ora) – is it possible that we would make the changes and then not be able to easily tell that the defaults have changed (I believe that changed hidden parameters will show in V$PARAMETER)?

Kevin makes several good points also (I suspect that he has more points to make, and that he is giving other people an opportunity to participate in the discussion).

By: kevinclosson

kevinclosson — Fri, 18 Jun 2010 16:41:46 +0000

I don’t think it is useful to recommend investigating 3 hidden parameters that have dubious impact in modern Oracle Database revisions. I understand that investigation is one thing, but to what end? If you start plopping these sorts of tunables in your init.ora and subsequently have any problems at all Support is likely to start by yanking out the settings.