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Quand utiliser le synchronuous I/O

En anglais :

You would be keen to disable asynchronous I/O  when you see high average_wait on event db_file_parallel_wait. Other reason for turning it off will be synchronous I/O is more reliable.

SQL> select event,average_wait from v$system_event where event like 'db file parallel write';
EVENT                                                            AVERAGE_WAIT
---------------------------------------------------------------- ------------
db file parallel write                                                    28.2  [ centi seconds]

This is not a very good ASYNCH I/O. Try Synchronous I/O


Other parameters to affect write (as well as read) is dbwriter_processes. When asynchronous I/O operations are slower in operating system in comparison to synchronous I/O then turn off asynchronous I/O by setting disk_asynch_io to false and set multiple db writer processes by increasing dbwriter_processes values from 1 to 2,3 or 4  suitable value to your system. Alternate is  incrase  dbwr_io_slaves from 0 to 2,3,4 suitable value.


Note 1: Asynchronous I/O operations are more prone to block corruptions than synchronous operations so many DBAs disable it and follow practice as mentioned in above paragraph. So if you do not have standby databases and oracle 11g then which autoamatically recovers corrupted block on primary then you would not want asynchronous I/O
Note 2: For 11g R2 for tuning purpose, the “db file async I/O submit” should be treated as   “db file parallel write” in previous releases.

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