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    Oracle Database Tips by Donald Burleson

Tuning the DBWR Process 

The DBWR process manages the buffer cache. In this capacity, it writes filled buffers from the buffer cache in the SGA to the disks. Obviously, a properly tuned DBWR process will be the first step in tuning I/O for the Oracle system. The DBWR process, as described in the section on UTLBSTAT and UTLESTAT, uses the hidden INIT.ORA parameters _DB_BLOCK_WRITE_BATCH and _DB_BLOCK_MAX_SCAN_CNT in Oracle8 to determine when it should write used, or dirty, buffers to the disk, thus freeing them for further use. DBWR triggers on the following conditions: 

  1. A user process writes a used buffer to the dirty buffer list and finds it is _DB_BLOCK_WRITE_BATCH / 2 long. 

  2. A user process searches _DB_BLOCK_MAX_SCAN_CNT buffers without finding a clean one.

  3. The  DBWR has been inactive for three seconds. 

  4. When a checkpoint occurs, LGWR signals DBWR to trigger it to write.     

The DBWR writes out _DB_BLOCK_WRITE_BATCH buffers each time it is triggered. If there aren't that many buffers in the dirty buffer list, the buffers on the LRU list are written until _DB_BLOCK_WRITE_BATCH buffers are written.


In Oracle8i and Oracle9i, you can no longer tune the parameters mentioned above because they have been deprecated. In Oracle8i and Oracle9i, the parameter _DB_WRITER_MAX_WRITES controls the maximum number of outstanding I/Os that a database writer can issue, but you should not touch this parameter unless instructed to by Oracle Support.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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