92 lines
2.8 KiB
Groff
92 lines
2.8 KiB
Groff
.\" Copyright (c) 2000 Sheldon Hearn <sheldonh@FreeBSD.org>
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.\" 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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.\"
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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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.\" $FreeBSD$
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.\"
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.Dd July 14, 2000
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.Dt SYNCER 4
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.Os
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.Sh NAME
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.Nm syncer
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.Nd filesystem synchronizer kernel process
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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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kernel process helps protect the integrity of disk volumes
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by flushing volatile cached filesystem data to disk.
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.Pp
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The kernel places all
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.Xr vnode 9 Ns 's
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in a number of queues.
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The
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.Nm
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process works through the queues
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in a round-robin fashion,
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usually processing one queue per second.
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For each
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.Xr vnode 9
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on that queue,
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the
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.Nm
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process forces a write out to disk of its dirty buffers.
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.Pp
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The usual delay between the time buffers are dirtied
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and the time they are synced
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is controlled by the following
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.Xr sysctl 8
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tunable variables:
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.Bl -column "filedelayXXXX" "DefaultXX" "DescriptionXX"
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.It Em Variable Ta Em Default Ta Em Description
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.It Va kern.filedelay Ta 30 Ta "time to delay syncing files"
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.It Va kern.dirdelay Ta 29 Ta "time to delay syncing directories"
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.It Va kern.metadelay Ta 28 Ta "time to delay syncing metadata"
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.El
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.Sh SEE ALSO
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.Xr sync 2 ,
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.Xr fsck 8 ,
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.Xr sync 8 ,
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.Xr sysctl 8
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.Sh HISTORY
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The
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.Nm
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process is a descendant of the
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.Sq update
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command, which appeared in
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.At v6 ,
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and was usually started by
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.Pa /etc/rc
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when the system went multi-user.
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A kernel initiated
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.Sq update
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process first appeared in
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.Fx 2.0 .
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.Sh BUGS
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It is possible on some systems that a
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.Xr sync 2
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occurring simultaneously with a crash may cause
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file system damage. See
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.Xr fsck 8 .
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