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Background Processes for an Instance
The most common background processes are shown in Figure 2-8 and vary from
Oracle release to release. Among the background processes in Oracle Database 11g
are the following:
Database Writer (DBWn)
Writes database blocks from the database buffer cache in the SGA to the datafiles
on disk. An Oracle instance can have up to 20 DBW processes to handle the
I/O load to multiple datafiles??”hence the notation DBWn. Most instances run
one DBW. DBW writes blocks out of the cache for two main reasons:
??? If Oracle needs to perform a checkpoint (i.e., to update the blocks of the
datafiles so that they ???catch up??? to the redo logs). Oracle writes the redo for
a transaction when it??™s committed, and later writes the actual blocks. Periodically,
Oracle performs a checkpoint to bring the datafile contents in line
with the redo that was written out for the committed transactions.
??? If Oracle needs to read blocks requested by users into the cache and there is
no free space in the buffer cache.


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