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  Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Buffer Cache Management

When an Oracle process requires data, it searches the buffer cache, finds the data blocks, and then uses the data. This is known as a 'cache hit'. If it cannot find the data, then it must be obtained from the data file. In this case, it finds a free buffer to accommodate the data block by scanning the LRU list, starting at the least-recently-used from the end of the list. The process searches either until it finds a free buffer or until it has searched the threshold limit of buffers.

When the user process is performing a full table scan, it reads the data blocks into buffers and places them on the LRU end instead of the MRU end of the LRU list. This is because a fully scanned table is usually needed only briefly and the blocks should be moved out quickly.

What Is a Dirty Block?

Whenever a server process changes or modifies a data block, it becomes a dirty block. Once a server process makes changes to the data block, the user may commit transactions, or transactions may not be committed for quite some time. In either case, the dirty block is not immediately written back to disk.

Writing dirty blocks to disk takes place under the following two conditions:

When a server process cannot find a clean, reusable buffer after scanning a threshold number of buffers, then the database writer process writes the dirty blocks to disk.

The above text is an excerpt from:

Oracle 10g Grid & Real Application Clusters
Oracle 10g Grid Computing with RAC
ISBN 0-9744355-4-6

by Mike Ault, Madhu Tumma


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