.\"	$NetBSD: sync.2,v 1.18 2017/05/14 12:30:37 wiz Exp $
.\"
.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  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. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
.\"
.\"     @(#)sync.2	8.1 (Berkeley) 6/4/93
.\"
.Dd March 25, 2009
.Dt SYNC 2
.Os
.Sh NAME
.Nm sync
.Nd "synchronize disk block in-core status with that on disk"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft void
.Fn sync void
.Sh DESCRIPTION
The
.Fn sync
function forces a write of dirty (modified) buffers
in the block buffer cache out
to disk.
The kernel keeps this information in core to reduce
the number of disk I/O transfers required by the system.
As information in the cache is lost after a system crash,
kernel thread
.Nm ioflush
ensures that dirty buffers are synced to disk
eventually.
By default, a dirty buffer is synced after 30 seconds,
but some file systems exploit
.Nm ioflush
features to sync directory data and metadata faster
(after 15 and 10 seconds, respectively).
.Pp
The function
.Xr fsync 2
may be used to synchronize individual file descriptor
attributes.
.Sh CAUTIONS
Many modern disks contain write-back caches.
In theory
.Fn sync
flushes these.
In practice there are many possible ways for this mechanism to go
astray.
It is prudent (where possible) to allow a few seconds after syncing
for everything to settle before e.g. turning off the power.
.Pp
It may also be desirable to use
.Xr dkctl 8
or
.Xr scsictl 8
to disable the write-back cache entirely.
.Sh SEE ALSO
.Xr fsync 2 ,
.Xr dkctl 8 ,
.Xr scsictl 8 ,
.Xr sync 8
.Sh HISTORY
A
.Fn sync
function call appeared in
.At v6 .
.Pp
Historically,
.Fn sync
would schedule buffers for writing but not actually wait for the
writes to finish.
It was necessary to issue a second or sometimes a third call to ensure
that all buffers had in fact been written out.
In
.Nx ,
.Fn sync
does not return until all buffers have been written.