db_writer_processes and dbwr_io_slaves

gdpr-dba

官方文档里有这么一句话：“Multiple DBWRs and DBWR IO slaves cannot coexist. ”，但是我并不是很理解，因为这两者感觉不应该冲突吧，db_writer_processes是控制dbwr进程的数量，dbwr_io_slaves是控制每个dbwr进程是否使用从属进程，有冲突吗？

Maclean Liu(刘相兵

In the case of multiple DBWR processes, each DBWR will manage its own LRU sets.
Multiple DBWRs are implemented with the parameter DB_WRITER_PROCESSES.
        In Oracle9i, Release 2, KCBMAXDBWRS, which defines max value for DB_WRITER_PROCESSES has been increased to 20.

Keyword: DBWR_IO_SLAVES
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http://docs.oracle.com/cd/B19306 ... 1/instance_tune.htm
10.3.9.3.2 DBWR_IO_SLAVES
If it is not practical to use multiple DBWR processes, then Oracle provides a facility whereby the I/O load can be distributed over multiple slave processes. The DBWR process is the only process that scans the buffer cache LRU list for blocks to be written out. However, the I/O for those blocks is performed by the I/O slaves. The number of I/O slaves is determined by the parameter DBWR_IO_SLAVES.

http://docs.oracle.com/cd/B19306 ... 7/initparams058.htm
DBWR_IO_SLAVES



当无法使用多DBWR PROCESS或Async IO异步的情况下才会不得不采用I/O slave ， I/O Slave的总数取决于参数DBWR_IO_SLAVES


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.