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head/share/man/man4/aio.4
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| .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) | .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) | ||||
| .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT | .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT | ||||
| .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY | .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY | ||||
| .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF | .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF | ||||
| .\" SUCH DAMAGE. | .\" SUCH DAMAGE. | ||||
| .\" | .\" | ||||
| .\" $FreeBSD$ | .\" $FreeBSD$ | ||||
| .\" | .\" | ||||
| .Dd July 15, 2016 | .Dd July 21, 2016 | ||||
| .Dt AIO 4 | .Dt AIO 4 | ||||
| .Os | .Os | ||||
| .Sh NAME | .Sh NAME | ||||
| .Nm aio | .Nm aio | ||||
| .Nd asynchronous I/O | .Nd asynchronous I/O | ||||
| .Sh DESCRIPTION | .Sh DESCRIPTION | ||||
| The | The | ||||
| .Nm | .Nm | ||||
| facility provides system calls for asynchronous I/O. | facility provides system calls for asynchronous I/O. | ||||
| However, asynchronous I/O operations are only enabled for certain file | Asynchronous I/O operations are not completed synchronously by the | ||||
| types by default. | calling thread. | ||||
| Asynchronous I/O operations for other file types may block an AIO daemon | Instead, the calling thread invokes one system call to request an | ||||
| indefinitely resulting in process and/or system hangs. | asynchronous I/O operation. | ||||
| Asynchronous I/O operations can be enabled for all file types by setting | The status of a completed request is retrieved later via a separate | ||||
| system call. | |||||
| .Pp | |||||
| Asynchronous I/O operations on some file descriptor types may block an | |||||
| AIO daemon indefinitely resulting in process and/or system hangs. | |||||
| Operations on these file descriptor types are considered | |||||
| .Dq unsafe | |||||
| and disabled by default. | |||||
| They can be enabled by setting | |||||
| the | the | ||||
| .Va vfs.aio.enable_unsafe | .Va vfs.aio.enable_unsafe | ||||
| sysctl node to a non-zero value. | sysctl node to a non-zero value. | ||||
| .Pp | .Pp | ||||
| Asynchronous I/O operations on sockets and raw disk devices do not block | Asynchronous I/O operations on sockets and raw disk devices do not block | ||||
| indefinitely and are enabled by default. | indefinitely and are always enabled. | ||||
| .Pp | .Pp | ||||
| The | The | ||||
| .Nm | .Nm | ||||
| facility uses kernel processes | facility uses kernel processes | ||||
| (also known as AIO daemons) | (also known as AIO daemons) | ||||
| to service most asynchronous I/O requests. | to service most asynchronous I/O requests. | ||||
| These processes are grouped into pools containing a variable number of | These processes are grouped into pools containing a variable number of | ||||
| processes. | processes. | ||||
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