.\" Copyright (c) 2000 Sheldon Hearn <sheldonh@FreeBSD.org>
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.\" $FreeBSD: src/share/man/man4/syncer.4,v 1.3.2.5 2001/07/21 09:16:47 schweikh Exp $
.\" $DragonFly: src/share/man/man4/syncer.4,v 1.2 2003/06/17 04:36:59 dillon Exp $
.\"
.Dd July 14, 2000
.Dt SYNCER 4
.Os
.Sh NAME
.Nm syncer
.Nd filesystem synchronizer kernel process
.Sh SYNOPSIS
.Nm
.Sh DESCRIPTION
The
.Nm
kernel process helps protect the integrity of disk volumes
by flushing volatile cached filesystem data to disk.
.Pp
The kernel places all
.Xr vnode 9 Ns 's
in a number of queues.
The
.Nm
process works through the queues
in a round-robin fashion,
usually processing one queue per second.
For each
.Xr vnode 9
on that queue,
the
.Nm
process forces a write out to disk of its dirty buffers.
.Pp
The usual delay between the time buffers are dirtied
and the time they are synced
is controlled by the following
.Xr sysctl 8
tunable variables:
.Bl -column "filedelayXXXX" "DefaultXX" "DescriptionXX"
.It Em Variable Ta Em Default Ta Em Description
.It Va kern.filedelay Ta 30 Ta "time to delay syncing files"
.It Va kern.dirdelay Ta 29 Ta "time to delay syncing directories"
.It Va kern.metadelay Ta 28 Ta "time to delay syncing metadata"
.El
.Sh SEE ALSO
.Xr sync 2 ,
.Xr fsck 8 ,
.Xr sync 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
process is a descendant of the
.Sq update
command, which appeared in
.At v6 ,
and was usually started by
.Pa /etc/rc
when the system went multi-user.
A kernel initiated
.Sq update
process first appeared in
.Fx 2.0 .
.Sh BUGS
It is possible on some systems that a
.Xr sync 2
occurring simultaneously with a crash may cause
file system damage.  See
.Xr fsck 8 .