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.
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©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>It is no problem that dirty buffers reside on an LRU chain. In fact, it would be a massive performance problem if they couldn't reside on an LRU chain. One of the objectives of the LRU chains is to help keep popular buffers in the cache, and many dirty buffers are also very popular. During a database checkpoint though, every dirty buffer does get written to disk and becomes free once again.
p><p>Figure 6-15. This is an example of Oracle's LRU chains. There are 2 LRU chain latches and 6 LRU chains, protecting 20 buffer headers. Each buffer header is associated with a single free, pinned, or dirty buffer.
p><p>Figure 6-15 is a good scaled-down diagram of Oracle LRU chains. Production Oracle systems usually have less than 50 LRU chains; most have between 8 and 16 chains. The hidden instance parameter _db_block_lru_latches shows how many LRU chains the instance is using. As with the CBCs, each LRU chain latch controls serialization for a group of LRU chains. Each of the Figure 6-15 LRU chains is protected by either latch CS 900 or CS 910. Just as with the CBC structure, each LRU chain is entirely made up of buffer headers. In fact, each of the buffer headers in the Figure 6-15 LRU chains is also linked to one of the CBCs! Each buffer header is labeled with the letter F, P, or D to signify it is free, pinned, or dirty. As I mentioned, in most production Oracle systems, each LRU chain links thousands of buffer headers.
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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