DB_WRITER_PROCESSES = 2 * number of CPU cores
Multiple DBWR processes is preferred over the use of I/O slaves. I/O slaves should only be used if multiple DBWR processes cannot be configured.
It is DBWR’s responsibility to make free buffers available by writing modified blocks to the datafiles.
WHERE event LIKE 'db file %'
OR event = 'free buffer waits'
OR event = 'write complete waits'
ORDER BY time_waited DESC
TOTAL TIME AVG
EVENT WAITS WAITED WAIT
----------------------- ------ ------- -----
free buffer waits 194278 4488038 23.10
db file sequential read 588805 2900049 4.93
db file parallel write 34667 119035 3.43
db file scattered read 19283 10242 0.53
write complete waits 175 5481 31.32
db file single write 378 1261 3.34
Firstly, the operating system and database instance must be configured to make non-blocking I/O (asynchronous I/O) available to DBWR.
Secondly, the datafiles must be spread over enough disks, controllers and I/O busses to ensure that there are no hot spots in the I/O subsystem.
This I/O tuning typically involves the use of striping, and commonly involves the use of raw datafiles also.
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