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.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<N> .
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<N> .
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 .
