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+.\"	$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.