UNIX Filesystems: Evolution, Design, and Implementation

Asynchronous I/O

By issuing an I/O asynchronously, an application can continue with other work rather than waiting for the I/O to complete. There have been numerous different implementations of asynchronous I/O (commonly referred to as async I/O) over the years. This section will describe the interfaces as supported by the Single UNIX Specification.

As an example of where async I/O is commonly used, consider the Oracle database writer process (DBWR), one of the main Oracle processes; its role is to manage the Oracle buffer cache, a user-level cache of database blocks. This involves responding to read requests and writing dirty (modified) buffers to disk.

In an active database, the work of DBWR is complicated by the fact that it is constantly writing dirty buffers to disk in order to allow new blocks to be read. Oracle employs two methods to help alleviate some of the performance bottlenecks. First, it supports multiple DBWR processes (called DBWR slave processes); the second option, which greatly improves throughput, is through use of async I/O. If I/O operations are being performed asynchronously, the DBWR processes can be doing other work, whether flushing more buffers to disk, reading data from disk, or other internal functions.

All of the Single UNIX Specification async I/O operations center around an I/O control block defined by the aiocb structure as follows:

struct aiocb { int aio_fildes; /* file descriptor */ off_t aio_offset; /* file offset */ volatile void *aio_buf; ...