1.\" Copyright (c) 2000 Sheldon Hearn <sheldonh@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD: src/share/man/man4/syncer.4,v 1.3.2.5 2001/07/21 09:16:47 schweikh Exp $ 26.\" $DragonFly: src/share/man/man4/syncer.4,v 1.2 2003/06/17 04:36:59 dillon Exp $ 27.\" 28.Dd July 14, 2000 29.Dt SYNCER 4 30.Os 31.Sh NAME 32.Nm syncer 33.Nd filesystem synchronizer kernel process 34.Sh SYNOPSIS 35.Nm 36.Sh DESCRIPTION 37The 38.Nm 39kernel process helps protect the integrity of disk volumes 40by flushing volatile cached filesystem data to disk. 41.Pp 42The kernel places all 43.Xr vnode 9 Ns 's 44in a number of queues. 45The 46.Nm 47process works through the queues 48in a round-robin fashion, 49usually processing one queue per second. 50For each 51.Xr vnode 9 52on that queue, 53the 54.Nm 55process forces a write out to disk of its dirty buffers. 56.Pp 57The usual delay between the time buffers are dirtied 58and the time they are synced 59is controlled by the following 60.Xr sysctl 8 61tunable variables: 62.Bl -column "filedelayXXXX" "DefaultXX" "DescriptionXX" 63.It Em Variable Ta Em Default Ta Em Description 64.It Va kern.filedelay Ta 30 Ta "time to delay syncing files" 65.It Va kern.dirdelay Ta 29 Ta "time to delay syncing directories" 66.It Va kern.metadelay Ta 28 Ta "time to delay syncing metadata" 67.El 68.Sh SEE ALSO 69.Xr sync 2 , 70.Xr fsck 8 , 71.Xr sync 8 , 72.Xr sysctl 8 73.Sh HISTORY 74The 75.Nm 76process is a descendant of the 77.Sq update 78command, which appeared in 79.At v6 , 80and was usually started by 81.Pa /etc/rc 82when the system went multi-user. 83A kernel initiated 84.Sq update 85process first appeared in 86.Fx 2.0 . 87.Sh BUGS 88It is possible on some systems that a 89.Xr sync 2 90occurring simultaneously with a crash may cause 91file system damage. See 92.Xr fsck 8 . 93