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.

Note

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.

See Code Depot

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