103 lines
3.3 KiB
Groff
103 lines
3.3 KiB
Groff
.\" $NetBSD: sync.2,v 1.18 2017/05/14 12:30:37 wiz Exp $
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.\"
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.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)sync.2 8.1 (Berkeley) 6/4/93
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.\"
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.Dd March 25, 2009
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.Dt SYNC 2
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.Os
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.Sh NAME
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.Nm sync
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.Nd "synchronize disk block in-core status with that on disk"
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft void
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.Fn sync void
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.Sh DESCRIPTION
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The
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.Fn sync
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function forces a write of dirty (modified) buffers
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in the block buffer cache out
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to disk.
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The kernel keeps this information in core to reduce
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the number of disk I/O transfers required by the system.
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As information in the cache is lost after a system crash,
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kernel thread
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.Nm ioflush
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ensures that dirty buffers are synced to disk
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eventually.
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By default, a dirty buffer is synced after 30 seconds,
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but some file systems exploit
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.Nm ioflush
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features to sync directory data and metadata faster
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(after 15 and 10 seconds, respectively).
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.Pp
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The function
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.Xr fsync 2
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may be used to synchronize individual file descriptor
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attributes.
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.Sh CAUTIONS
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Many modern disks contain write-back caches.
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In theory
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.Fn sync
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flushes these.
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In practice there are many possible ways for this mechanism to go
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astray.
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It is prudent (where possible) to allow a few seconds after syncing
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for everything to settle before e.g. turning off the power.
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.Pp
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It may also be desirable to use
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.Xr dkctl 8
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or
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.Xr scsictl 8
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to disable the write-back cache entirely.
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.Sh SEE ALSO
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.Xr fsync 2 ,
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.Xr dkctl 8 ,
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.Xr scsictl 8 ,
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.Xr sync 8
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.Sh HISTORY
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A
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.Fn sync
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function call appeared in
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.At v6 .
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.Pp
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Historically,
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.Fn sync
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would schedule buffers for writing but not actually wait for the
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writes to finish.
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It was necessary to issue a second or sometimes a third call to ensure
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that all buffers had in fact been written out.
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In
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.Nx ,
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.Fn sync
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does not return until all buffers have been written.
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