Oracle Database - Database Writer Process (DBWn)

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Oracle Database - Database Writer Process (DBWn)

About

Database writer process (DBWn) is a background process that writes buffers in the database buffer cache to data files.

Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate data files all at once (in bulk mode), as determined by the background process database writer process (DBWn).

The disk write can happen before or after the commit. See Database Buffer Cache.

After a COMMIT, the database writes the redo buffers to disk but does not immediately write data blocks to disk. Instead, database writer (DBWn) performs lazy writes in the background.

Write

The database writer (DBWn) process periodically writes cold, dirty buffers to disk. DBWn writes buffers in the following circumstances:

  • A server process cannot find clean buffers for reading new blocks into the database buffer cache.

As buffers are dirtied, the number of free buffers decreases. If the number drops below an internal threshold, and if clean buffers are required, then server processes signal DBWn to write.

The database uses the LRU to determine which dirty buffers to write. When dirty buffers reach the cold end of the LRU, the database moves them off the LRU to a write queue. DBWn writes buffers in the queue to disk, using multiblock writes if possible. This mechanism prevents the end of the LRU from becoming clogged with dirty buffers and allows clean buffers to be found for reuse.

  • The database must advance the checkpoint, which is the position in the redo thread from which instance recovery must begin.
  • Tablespaces are changed to read-only status or taken offline.

Documentation / Reference





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