Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage

Direct I/O

To solve all these problems, the direct I/O is highly desired. The way in which direct I/O is enabled varies from one system to another and also depends on the type of file system type. In some cases it is sufficient to set the filesystemio_options parameter; in some cases a file system mount option is required; and direct I/O can also be configured on a file-by-file basis using special operating system commands. The use of ASM eliminates need for direct I/O configuration.

Asynchronous I/O

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 result of the read/write operation, if success or failure, is understood 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.

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