aio

AIO(4)                    BSD Kernel Interfaces Manual                    AIO(4)

NAME
     aio — asynchronous I/O

DESCRIPTION
     The aio 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.

     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 “unsafe” and disabled by
     default.  They can be enabled by setting the vfs.aio.enable_unsafe sysctl
     node to a non-zero value.

     Asynchronous I/O operations on sockets, raw disk devices, and regular files
     on local filesystems do not block indefinitely and are always enabled.

     The aio 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.

     One pool of AIO daemons is used to service asynchronous I/O requests for
     sockets.  These processes are named “soaiod<N>”.  The following sysctl
     nodes are used with this pool:

     kern.ipc.aio.num_procs
             The current number of processes in the pool.

     kern.ipc.aio.target_procs
             The minimum number of processes that should be present in the pool.

     kern.ipc.aio.max_procs
             The maximum number of processes permitted in the pool.

     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.

     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
     “aiod<N>”.  The following sysctl nodes are used with this pool:

     vfs.aio.num_aio_procs
             The current number of processes in the pool.

     vfs.aio.target_aio_procs
             The minimum number of processes that should be present in the pool.

     vfs.aio.max_aio_procs
             The maximum number of processes permitted in the pool.

     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.

     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.

     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:

     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
             aio_return(2) or aio_waitcomplete(2) are not counted against this
             limit.

     vfs.aio.num_buf_aio
             The number of queued asynchronous I/O requests for raw disks
             system-wide.

     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.

     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
             aio_return(2) or aio_waitcomplete(2).

     vfs.aio.num_queue_count
             The number of outstanding asynchronous I/O requests system-wide.

     vfs.aio.max_aio_queue
             The maximum number of outstanding asynchronous I/O requests
             permitted system-wide.

     Asynchronous I/O control buffers should be zeroed before initializing
     individual fields.  This ensures all fields are initialized.

     All asynchronous I/O control buffers contain a sigevent structure in the
     aio_sigevent field which can be used to request notification when an
     operation completes.

     For SIGEV_KEVENT notifications, the sigevent s sigev_notify_kqueue field
     should contain the descriptor of the kqueue that the event should be
     attached to, its sigev_notify_kevent_flags field may contain EV_ONESHOT,
     EV_CLEAR, and/or EV_DISPATCH, and its sigev_notify field should be set to
     SIGEV_KEVENT.  The posted kevent will contain:

     Member    Value
     ident     asynchronous I/O control buffer pointer
     filter    EVFILT_AIO
     flags     EV_EOF
     udata     value stored in aio_sigevent.sigev_value

     For SIGEV_SIGNO and SIGEV_THREAD_ID notifications, the information for the
     queued signal will include SI_ASYNCIO in the si_code field and the value
     stored in sigevent.sigev_value in the si_value field.

     For SIGEV_THREAD notifications, the value stored in
     aio_sigevent.sigev_value is passed to the
     aio_sigevent.sigev_notify_function as described in sigevent(3).

SEE ALSO
     aio_cancel(2), aio_error(2), aio_read(2), aio_return(2), aio_suspend(2),
     aio_waitcomplete(2), aio_write(2), lio_listio(2), sigevent(3), sysctl(8)

HISTORY
     The aio facility appeared as a kernel option in FreeBSD 3.0.  The aio
     kernel module appeared in FreeBSD 5.0.  The aio facility was integrated
     into all kernels in FreeBSD 11.0.

BSD                               June 22, 2017                              BSD