MULTIPLE DBWn PARAMETERE DEFINE BECAUSE FREE BUFFER WAITS IN (DB BUFFER CACHE)

This means we are waiting for a free buffer but there are none available in the cache because there are too many dirty buffers in the cache. Causes :

1. Buffer cache is too small

Solutn: - Use AMM/ASMM
- Increase Db buffer cache size as per
- ADDM recommendation
- v$db_cache_advice

2. DBWR is slow in writing modified buffers to disk and is unable to keep up to the write requests.
Solutn: - Increase I/O bandwidth by striping the datafiles.
- If asynchronous I/O is supported .
--Enable asynchronous I/O (DISK_ASYNCH_IO=true)
---- if multiple CPU's are there,
      Increase the no. of database writers (DB_WRITER_PROCESSES)
           else
      Configure I/O slaves (set DBWR_IO_SLAVES to non zero )
else
Configure I/O slaves (set DBWR_IO_SLAVES to non zero )

3. Large sorts and full table scans are filling the cache with modified blocks faster than the DBWR is able to write to disk.
Solutn: - Pre-sorting or reorganizing data can help

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WHEN THE SGA SIZE IS MORE CONSUME AND AT A TIME ONE DBWn PROCESS ,ALL BUFFER CACHE DATA IS
WRITE ON DATAFILE BUT IN CASE YOU WILL EXECUTE SOME QUERIES BUT PROCESS IS NOT ALLOCATED
THE SPACE ON BUFFER CACHE ,SO MULTIPLE DBWn IS WORK FAST TWO OR MORE PROCESS IS WRITE A DATA
ON DATAFILE PARALLELY AND PROCESS ALLOCATE A SPACE ON BUFFER CACHE.

PARAMETER IS DEFINE ON THIS FILE:-
( Oracle I_O Slave Waits dbwr parallel DML   )

SHOW PARAMETER DB_WRITER_%

SHOW PARAMETER DBWR_IO_SLAVES

Parameter type Integer

Default value 0

Parameter class Static

Range of values 0 to operating system-dependent


DBWR_IO_SLAVES is relevant only on systems with only one database writer process (DBW0). It specifies the number of I/O server processes used by the DBW0 process. The DBW0 process and its server processes always write to disk. By default, the value is 0 and I/O server processes are not used.

If you set DBWR_IO_SLAVES to a nonzero value, the number of I/O server processes used by the ARCH and LGWR processes is set to 4. However, the number of I/O server processes used by Recovery Manager is set to 4 only if asynchronous I/O is disabled (either your platform does not support asynchronous I/O or disk_asynch_io is set to false.

Typically, I/O server processes are used to simulate asynchronous I/O on platforms that do not support asynchronous I/O or that implement it inefficiently. However, you can use I/O server processes even when asynchronous I/O is being used. In that case the I/O server processes will use asynchronous I/O.

I/O server processes are also useful in database environments with very large I/O throughput, even if asynchronous I/O is enabled.

NOTE:-
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IT MEANS THE  "DBWR_IO_SLAVES"  INITIAL 2 OR 3 PROCESS ARE EXCUTES PARALLELY
IF THE  "DBWR_IO_SLAVES"  NOT INITIALIZED THE PROCESS ARE EXECUTED IN A QUEUE
IN THIS WORK ON  "DB_WRITER_PROCESS"  PARAMETER INITIAL FIRST .