.\"- .\" Copyright (c) 2002 Dag-Erling Coïdan Smørgrav .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" 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 .\" SUCH DAMAGE. .\" .Dd January 2, 2021 .Dt AIO 4 .Os .Sh NAME .Nm aio .Nd asynchronous I/O .Sh DESCRIPTION The .Nm facility provides system calls for asynchronous I/O. Asynchronous I/O operations are not completed synchronously by the calling thread. Instead, the calling thread invokes one system call to request an asynchronous I/O operation. 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 .Va vfs.aio.enable_unsafe sysctl node to a non-zero value. .Pp Asynchronous I/O operations on sockets, raw disk devices, and regular files on local filesystems do not block indefinitely and are always enabled. .Pp The .Nm facility uses kernel processes (also known as AIO daemons) to service most asynchronous I/O requests. These processes are grouped into pools containing a variable number of processes. Each pool will add or remove processes to the pool based on load. Pools can be configured by sysctl nodes that define the minimum and maximum number of processes as well as the amount of time an idle process will wait before exiting. .Pp One pool of AIO daemons is used to service asynchronous I/O requests for sockets. These processes are named .Dq soaiod . The following sysctl nodes are used with this pool: .Bl -tag -width indent .It Va kern.ipc.aio.num_procs The current number of processes in the pool. .It Va kern.ipc.aio.target_procs The minimum number of processes that should be present in the pool. .It Va kern.ipc.aio.max_procs The maximum number of processes permitted in the pool. .It Va kern.ipc.aio.lifetime The amount of time a process is permitted to idle in clock ticks. If a process is idle for this amount of time and there are more processes in the pool than the target minimum, the process will exit. .El .Pp A second pool of AIO daemons is used to service all other asynchronous I/O requests except for I/O requests to raw disks. These processes are named .Dq aiod . The following sysctl nodes are used with this pool: .Bl -tag -width indent .It Va vfs.aio.num_aio_procs The current number of processes in the pool. .It Va vfs.aio.target_aio_procs The minimum number of processes that should be present in the pool. .It Va vfs.aio.max_aio_procs The maximum number of processes permitted in the pool. .It Va vfs.aio.aiod_lifetime The amount of time a process is permitted to idle in clock ticks. If a process is idle for this amount of time and there are more processes in the pool than the target minimum, the process will exit. .El .Pp Asynchronous I/O requests for raw disks are queued directly to the disk device layer after temporarily wiring the user pages associated with the request. These requests are not serviced by any of the AIO daemon pools. .Pp Several limits on the number of asynchronous I/O requests are imposed both system-wide and per-process. These limits are configured via the following sysctls: .Bl -tag -width indent .It Va vfs.aio.max_buf_aio The maximum number of queued asynchronous I/O requests for raw disks permitted for a single process. Asynchronous I/O requests that have completed but whose status has not been retrieved via .Xr aio_return 2 or .Xr aio_waitcomplete 2 are not counted against this limit. .It Va vfs.aio.num_buf_aio The number of queued asynchronous I/O requests for raw disks system-wide. .It Va vfs.aio.max_aio_queue_per_proc The maximum number of asynchronous I/O requests for a single process serviced concurrently by the default AIO daemon pool. .It Va vfs.aio.max_aio_per_proc The maximum number of outstanding asynchronous I/O requests permitted for a single process. This includes requests that have not been serviced, requests currently being serviced, and requests that have completed but whose status has not been retrieved via .Xr aio_return 2 or .Xr aio_waitcomplete 2 . .It Va vfs.aio.num_queue_count The number of outstanding asynchronous I/O requests system-wide. .It Va vfs.aio.max_aio_queue The maximum number of outstanding asynchronous I/O requests permitted system-wide. .El .Pp Asynchronous I/O control buffers should be zeroed before initializing individual fields. This ensures all fields are initialized. .Pp All asynchronous I/O control buffers contain a .Vt sigevent structure in the .Va aio_sigevent field which can be used to request notification when an operation completes. .Pp For .Dv SIGEV_KEVENT notifications, the .Va sigevent .Ap s .Va sigev_notify_kqueue field should contain the descriptor of the kqueue that the event should be attached to, its .Va sigev_notify_kevent_flags field may contain .Dv EV_ONESHOT , .Dv EV_CLEAR , and/or .Dv EV_DISPATCH , and its .Va sigev_notify field should be set to .Dv SIGEV_KEVENT . The posted kevent will contain: .Bl -column ".Va filter" .It Sy Member Ta Sy Value .It Va ident Ta asynchronous I/O control buffer pointer .It Va filter Ta Dv EVFILT_AIO .It Va flags Ta Dv EV_EOF .It Va udata Ta value stored in .Va aio_sigevent.sigev_value .El .Pp For .Dv SIGEV_SIGNO and .Dv SIGEV_THREAD_ID notifications, the information for the queued signal will include .Dv SI_ASYNCIO in the .Va si_code field and the value stored in .Va sigevent.sigev_value in the .Va si_value field. .Pp For .Dv SIGEV_THREAD notifications, the value stored in .Va aio_sigevent.sigev_value is passed to the .Va aio_sigevent.sigev_notify_function as described in .Xr sigevent 3 . .Sh SEE ALSO .Xr aio_cancel 2 , .Xr aio_error 2 , .Xr aio_read 2 , .Xr aio_readv 2 , .Xr aio_return 2 , .Xr aio_suspend 2 , .Xr aio_waitcomplete 2 , .Xr aio_write 2 , .Xr aio_writev 2 , .Xr lio_listio 2 , .Xr sigevent 3 , .Xr sysctl 8 .Sh HISTORY The .Nm facility appeared as a kernel option in .Fx 3.0 . The .Nm kernel module appeared in .Fx 5.0 . The .Nm facility was integrated into all kernels in .Fx 11.0 .