We can configure more than one DBWn , if fact we can configure up to 36 DBWn.Optimally, the DBWn uses asynchronous I/O to write blocks to disk.With a asynchronous input output .DBWn gathers up a batch of blocks out;rather it goes back and collects the next batch to be written.As the operating system completes the writes, it asynchronously notifies DBWn that it completed the writes.This allows DBWn to work much faster than if it had to do everything serially.
DBWR is slower than LGWR!!
Database writer by definition writes out blocks scattered all over disk – DBWR does lots of scattered writes.When you do an update,you will be modifying index blocks that are stored here and there , and data blocks that are randomly distributed on the disk.LGWR on the other hand does a lot of sequential writes to the redo log this is an important distinction and one of the main reasons that Oracle has a redo log and the LGWR process as well as DBWn Process.Scattered writes are significantly slower than the sequential writes.
Before DBWn can write a modified buffer, all redo records associated with the changes to the buffer must be written to disk (the write-ahead protocol). If DBWn finds that some redo records have not been written, it signals LGWR to write the redo records to disk and waits for LGWR to complete writing the redo log buffer before it can write out the data buffers.
How DBWn Works:
When a buffer in the database buffer cache is modified, it is marked dirty.A cold buffer is a buffer that has not been recently used according to the least recently used (LRU) algorithm. The DBWn process writes cold, dirty buffers to disk so that user processes are able to find cold, clean buffers that can be used to read new blocks into the cache. As buffers are dirtied by user processes, the number of free buffers diminishes. If the number of free buffers drops too low, user processes that must read blocks from disk into the cache are not able to find free buffers. DBWn manages the buffer cache so that user processes can always find free buffers.By writing cold, dirty buffers to disk, DBWn improves the performance of finding free buffers while keeping recently used buffers resident in memory. For example, blocks that are part of frequently accessed small tables or indexes are kept in the cache so that they do not need to be read in again 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. If it is not specified by the user during startup, Oracle determines how to set DB_BLOCK_PROCESSES based on the number of CPUs and processor groups.The DBWn process writes dirty buffers to disk under the following conditions:
- When a server process cannot find a clean reusable buffer after scanning a threshold number of buffers, it signals DBWn to write. DBWn writes dirty buffers to disk asynchronously while performing other processing.
DBWn periodically writes buffers to advance the checkpoint, which is the position in the redo thread (log) from which instance recovery begins. This log position is determined by the oldest dirty buffer in the buffer cache.In all cases, DBWn performs batched (multiblock) writes to improve efficiency. The number of blocks written in a multiblock write varies by operating system.