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177ba18307
People still believe that it is essential to run sync(8) a couple of times before a reboot/halt. Document that this has not been necessary for a long time now. Reviewed by: imp, bcr, Pau Amma <pauamma@gundo.com> MFC after: 3 days Differential Revision: https://reviews.freebsd.org/D33233
100 lines
2.8 KiB
Groff
100 lines
2.8 KiB
Groff
.\"-
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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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.Dd April 25, 2024
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.Dt SYNC 8
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.Os
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.Sh NAME
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.Nm sync
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.Nd force completion of pending disk writes (flush cache)
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.Sh SYNOPSIS
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.Nm
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.Sh DESCRIPTION
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The
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.Nm
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utility
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can be called to ensure that all disk writes have been completed before the
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processor is halted in a way not suitably done by
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.Xr reboot 8
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or
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.Xr halt 8 .
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Generally, it is preferable to use
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.Xr reboot 8
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or
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.Xr halt 8
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to shut down the system,
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as they may perform additional actions
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such as resynchronizing the hardware clock
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and flushing internal caches before performing a final
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.Nm .
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.Pp
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The
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.Nm
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utility utilizes the
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.Xr sync 2
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function call.
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.Sh SEE ALSO
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.Xr fsync 2 ,
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.Xr sync 2 ,
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.Xr syncer 4 ,
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.Xr halt 8 ,
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.Xr reboot 8
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.Sh HISTORY
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A
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.Nm
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utility appeared in
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.At v4 .
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.Pp
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On systems older than
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.Bx 4.0 ,
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commands like
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.Xr reboot 8
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and
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.Xr halt 8
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were unavailable.
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The shutdown procedure involved running
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.Nm ,
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waiting for the lights to stop,
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and turning off the machine.
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.Pp
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Issuing three separate
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.Nm
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commands (one line each) was a placebo that would generally suffice in
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.At v7
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machines that were otherwise quiesced systems.
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It replaced the one-per-line
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.Nm
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as a substitute for waiting.
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.Pp
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.Bx 4.0
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introduced
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.Xr reboot 2
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and
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.Xr sync 2
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which rendered this trick obsolete.
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