Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage

Asynchronous I/O

The performance of asynchronous I/O depends much on if the kernelized asynchronous I/O or threaded asynchronous I/O is used.

• For kernelized asynchronous I/O, the kernel allocates an asynchronous I/O request data structure and calls an entry point in the device driver to set up the asynchronous I/O request. The device driver then queues the physical I/O operation and returns control to calling process. When the physical I/O operation has completed, the hardware generates an interrupt to a CPU. The CPU switches into interrupt service context and calls the device driver's interrupt service routine to update the asynchronous I/O request data structure and possibly to signal the calling process with SIGIO.
   

• The threaded implementation of asynchronous I/O uses the kernel's lightweight process functionality to simulate asynchronous I/O by performing multiple synchronous I/O requests in distinct threads. 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. For this reason, using kernelized asynchronous I/O is a preferred method.

Kernelized Asynchronous I/O, popularly known as KIO, is only available if the underlying file system uses Oracle Disk Manager (ODM) API, Veritas Quick I/O, or a similar product that routes the I/O via a pseudo device driver that can serve as the locus for asynchronous I/O request completion. In addition, KIO is available if you use the raw partitions. Many operating systems also require special configuration of device files, device drivers and kernel parameters to enable and tune kernelized asynchronous I/O. It is definitely a complex configuration to achieve Asynchronous IO.

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