Oracle Internals Notes

Threaded Asynchronous I/O

Because kernelized asynchronous I/O is only available for raw devices and Quick I/O files, some operating systems provide an alternative code path in their implementation of the aio_read() and aio_write() system calls that can be used to simulate asynchronous I/O on ordinary file system based files. For every concurrent threaded asynchronous I/O operation, a light-weight process is allocated or spawned to perform an ordinary synchronous (blocking) I/O operation. This achieves I/O parallelism at the expense of additional CPU usage associated with thread creation and extra context switching overheads. If threaded asynchronous I/O is used very intensively, these costs can add as much as 5% to system CPU usage.

The cost of threaded asynchronous I/O is such that it is only beneficial if an application program is not able to use multiple processes to simulate asynchronous I/O, as for example Oracle's database writer can do.

The following operating systems support threaded asynchronous I/O: Solaris, AIX, Windows NT/2000, Linux, Irix, Tru64 Unix ...
The following operating systems do not support threaded asynchronous I/O: HP-UX, Reliant Unix, ...

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