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Oracle Asynchronous I/O (aio) tips

Oracle I/O Tips by Burleson Consulting

Question:  What is asynchronous I/O and how is asynchronous I/O different from standard I/O?

Answer:  In a nutshell, asynchronous I/O is where the backend I/O subsystem (e.g. a storage array) "acknowledges" that a write has been completed while it is still waiting for a permanent write in the storage array.  This is called a "false ack", the way that Oracle can perform high volumes of writes when working with a vendor storage array.

When a system process attempts to read or write using the normal synchronous read() or write() system calls, then it must wait until the physical I/O is completed. Once the success or failure of the read/write operation is known, the process finishes the task. During this time, the execution of the process is blocked while it waits for the results of the system call. This is synchronous or blocking I/O.

However, the desired method is Asynchronous I/O which indicates that it is a Non-blocking I/O. If the process instead uses the asynchronous aio_read() or aio_write() system calls, then the system call will return immediately once the I/O request has been passed down to the hardware or queued in the operating system, typically before the physical I/O operation has even begun. It can continue executing and then receive the results of the I/O operation later, once they are available. Thus it is asynchronous or non-blocking I/O.

Asynchronous I/O enables write intensive processes like Oracle's DBWn to make full use of the I/O bandwidth of the hardware, by queuing I/O requests to distinct devices in quick succession so that they can be processed largely in parallel. Asynchronous I/O also allows processes performing compute intensive operations like sorts to pre-fetch data from disk before it is required so that the I/O and computation can occur in parallel.

The performance of asynchronous I/O is 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 the 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 light-weight 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. Also KIO is available if using 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 I/O. The best news is that the KIO is available with the ASM files automatically.


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