Database Writer Process (DBWn)

The server process records changes to rollback and data blocks in the buffer cache. Database Writer (DBWn) writes the dirty buffers from the database buffer cache to the data files. Although one database writer process is enough for most systems, you can configure additional processes through  from DBW1 to DBW9 and from DBWa to DBWj to improve write performance if your system modifies data heavily. When a buffer in the database buffer cache is changed, it is marked as a dirty buffer. A dirty buffer is a buffer that has not been recently used according to the least recently used . The DBWn process writes dirty buffers to disk so that user processes are able to find free buffers that can be used to write new blocks into the cache. If the number of free buffers are low in the cache and user processor are not able to find free blocks in the cache then  DBWn manages the buffer cache so that user processes can always find free buffers.
DBWn improves the performance because server process make changes only in buffer cache and DBWn manages the buffer cache to keep free buffers in the cache so that processor can find easily free buffer in cache and dirty buffer writes in to the data files. For example, blocks that are often  accessed small tables or indexes are kept in the cache so that there will be cache hit means there is no requirement to read these blocks from disk. The LRU algorithm keeps more frequently accessed blocks in the buffer cache so that when a buffer is written to disk, it is unlikely to contain data that will be useful soon.
The initialization parameter DB_WRITER_PROCESSES specifies the number of DBWn processes. The maximum number of DBWn processes is 20. The DBWn process writes dirty buffers to disk under the following conditions:
  • Incremental or normal checkpoint
  • The number of dirty buffers reaches a threshold value
  • A process scans a specified number of blocks when scanning for free buffers and cannot fine any.
  • Timeout occurs.
  • A ping request in Real Application Clusters environment.
  • Placing a normal or temporary tablespace offline.
  • Placing a tablespace in read only mode.
  • Dropping or Truncating a table.

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