Craig Shallahamer's Blog
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You were brought to this page based on an internet search
and as a free service to Oracle DBAs.
The text below is an except from the book,
Oracle Performance Firefighting, written by
Craig Shallahamer of
OraPub, Inc.
Figures and tables are not included on this page, only their reference.
To order the book in either print or PDF form, click
here.
©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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<p>Regardless of what you find, when experiencing significant log file sync waits, assume there is a rapid commit issue and find out why it is occurring. Understanding this will help you devise a better application and nonapplication solution strategy.
p><p>A log file sync wait event is commonly attributed to either a lack of IO or CPU resources, or both. If the bottleneck is both CPU and IO, it tends to quickly shift between the two, which when averaged over an interval, does not clearly show either one to be a problem. This can easily cloud your understanding of the situation, so you must be very careful.
p><p>Remain focused on the application commit times. If the log writer is pushing more IO (requirements) than the IO subsystem can absorb (capacity), the top wait event will be log file parallel write, with average wait times greater than 10 ms and high IO subsystem response times gathered from iostat or sar. If the IO subsystem is responding poorly, the server process posting the log file sync will indeed need to wait for the log writer background process to finish writing.
p>
©2009, 2010 by Craig Shallahamer. This is copyrighted material.
Please—Out of respect for those involved in the creation of the book and also for
their familes, we ask you to respect the copyright both in intent and deed. Thank you.
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