man/man4/aio.4

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.Dd March 1, 2016
.Dt AIO 4
.Os
.Sh NAME
.Nm aio
.Nd asynchronous I/O
.Sh DESCRIPTION
The
.Nm
facility provides system calls for asynchronous I/O.
However, asynchronous I/O operations are only enabled for certain file
types by default.
Asynchronous I/O operations for other file types may block an AIO daemon
indefinitely resulting in process and/or system hangs.
Asynchronous I/O operations can be enabled for all file types by setting
the
.Va vfs.aio.enable_unsafe
sysctl node to a non-zero value.
.Pp
Asynchronous I/O operations on sockets and raw disk devices do not block
indefinitely and are enabled by default.
.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
.Sh SEE ALSO
.Xr aio_cancel 2 ,
.Xr aio_error 2 ,
.Xr aio_read 2 ,
.Xr aio_return 2 ,
.Xr aio_suspend 2 ,
.Xr aio_waitcomplete 2 ,
.Xr aio_write 2 ,
.Xr lio_listio 2 ,
.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 .