NOLOGGING Option and log file sync Waits – What is Wrong with this Quote?

8 06 2011

June 8, 2011

I have not had published a “What is Wrong with this Quote” blog article for a couple of months, so I thought that I would publish a few of those types of articles.

The first quote is from the Alpha copy of the book “Oracle 11g Performance Tuning Recipes”, recipe 5-7 Resolving Log File Sync Waits.  As always, I try to minimize the amount of text that I post for a quote, but a sufficient amount so that the context of the quote is not completely lost.  The quote:

“As the log file sync wait event is caused by contention caused by the LGWR process, see if you can use the NOLOGGING option to get rid of these waits. Of course, in a production system, you can’t use the NOLOGGING option when the database is processing user requests, so this option is of limited use in most cases.”

“Although the database automatically calculates the value of the log_io_size parameter, you can specify a value for it, by issuing a command such as the following:

SQL> alter system set "_log_io_size"=1024000 scope=spfile;

System altered.                                 "

What, if anything, is wrong with the above quote?

Hint for the first half of the quote: AskTom, Documentation
Hint for the second half of the quote: The scale for the _log_io_size parameter is NOT bytes.  This article may also be helpful.

The point of blog articles like this one is not to insult authors who have spent thousands of hours carefully constructing an accurate and helpful book, but instead to suggest that readers investigate when something stated does not exactly match what one believes to be true.  It could be that the author “took a long walk down a short pier”, or that the author is revealing accurate information which simply cannot be found through other resources (and may in the process be directly contradicting information sources you have used in the past).  If you do not investigate in such cases, you may lose an important opportunity to learn something that could prove to be extremely valuable.