Writes modified blocks from the database buffer cache to the data files.
The database writer process (DBWn) reads the database buffer cache and writes modified buffers to data files. It also handles checkpoints, file open synchronization, and logging of Block Written records. DBWn also reads the Database Smart Flash Cache (Flash Cache) when Flash Cache is configured.
In many cases the blocks that DBWn writes are scattered throughout the disk, so the writes tend to be slower than the sequential writes performed by the log writer process(LGWR). DBWn performs multiblock writes when possible to improve efficiency. The number of blocks that are written in a multiblock write varies by operating system.
The DB_WRITER_PROCESSES initialization parameter specifies the number of database writer processes. There can be 1 to 100 database writer processes. The names of the first36 database writer processes are DBW0-DBW9 and DBWa-DBWz. The names of the 37th through 100th database writer processes are BW36-BW99. The database selects an appropriate default setting for the DB_WRITER_PROCESSES parameter or adjusts a user-specified setting based on the number of CPUs and processor groups.
