DB File Scattered Read Wait Event – What is Wrong with this Quote?

13 06 2010

June 13, 2010 (Modified June 14, 2010)

I found an interesting couple of lines in the June 2010 printing of the “Oracle Performance Firefighting” book.  This quote is from page 43 – I added the text in the square brackets [ ] to provide context for the quote:

Digging a little deeper [into the results of the readv Linux OS call that completed in 0.6ms], we can assert that all the requested blocks must have resided in memory (but not Oracle’s buffer cache memory, since Oracle needed to request them from the operating system). We can make this assertion because a physical spinning IO device cannot return 16 nonsequential blocks (the blocks could be scattered over many physical devices) in less than a single millisecond!

[One sentence removed to cut down the amount of typing for this quote - sentence basically states that Oracle assigns the time consumed to an Oracle wait event.]  Regardless of the operating system or the actual system call, Oracle gives a multiblock IO call the special name db file scattered read, because these multiple blocks can be scattered over the IO subsystem.”

This book was written by Craig Shallahamer, had six technical reviewers (I recognize the names of three), and was printed by Good Catch Publishing.

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

Edit June 14, 2010:
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.








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