Oracle Database Discussions

Hi Gurus,

So for my understanding is DBWR process will both read and write the blocks between db cache and disk.  Please let me know if there is any separate background process responsible for reading the blocks from the disk. Please correct me if i am wrong.

I have read the manual and it states,

Memory Structures and Processes

Database Writer (DBWR) Database Writer process (DBWR) writes buffers to datafiles. DBWR is an Oracle background process responsible for buffer cache management. For more information about the database buffer cache, see "The Database Buffer Cache" .

When a buffer in the buffer cache is modified, it is marked "dirty". The primary job of the DBWR process is to keep the buffer cache "clean" by writing dirty buffers to disk. As buffers are filled and 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. DBWR manages the buffer cache so that user processes can always find free buffers.

An LRU (least recently used) algorithm keeps the most recently used data blocks in memory and thus minimizes I/O. The database writer process (DBWR) keeps blocks that are used often, for example, blocks that are part of frequently accessed small tables or indexes, in the cache so that they do not need to be read in again from disk. To make room in the buffer cache for other blocks, DBWR removes blocks that are accessed infrequently (for example, blocks that are part of very large tables or leaf blocks from very large indexes) from the system global area (SGA). For information about leaf blocks, see "The Internal Structure of Indexes" http://docs.oracle.com/cd/A57673_01/DOC/server/doc/SCN73/ch5.htm#leafhttp://docs.oracle.com/cd/A57673_01/DOC/server/doc/SCN73/ch5.htm#leaf.

Regards,

Anand.