Craig Shallahamer's Blog
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You were brought to this page based on an internet search
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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>By default, every second, the MMNL background process wakes up and gathers active session information and writes the information in the ASH buffers, starting from where it last finished writing. This means the oldest ASH buffer resides just before-just ahead of-the next MMNL ASH buffer write. Since the ASH buffers form a ring structure, unless the records are archived, they will eventually be overwritten. Oracle will not allow this, so once an hour, or when the ASH ring structure is getting full, the MMNL and MMON background processes are woken and are involved in archiving ASH buffers to the AWR tables. Picture this archiving going clockwise, or forward in time.
</p><p>While it is common to hear about the ASH buffers being flushed, it is not a true flush, because the data is not cleared or erased. It remains, so we are more likely to find ASH data in memory. Oracle's AWR facility manages how much and the granularity of AWR data to keep before the rows are physically deleted from the AWR tables.
</p><p>When the v$active_session_history view is queried, you can think of it as starting at a specific period in the past and then progressing forward in time (moving clockwise). While most ASH queries pull data from a specific number of minutes in the past and move forward until the most recent ASH record, because ASH records are indexed and stored by time, the report start and stop time can be anything you wish, as long as the data is stored in the ASH ring structure.
</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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