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.\" $FreeBSD$ 26.\" 27.Dd July 14, 2000 28.Dt SYNCER 4 29.Os 30.Sh NAME 31.Nm syncer 32.Nd file system synchronizer kernel process 33.Sh SYNOPSIS 34.Nm 35.Sh DESCRIPTION 36The 37.Nm 38kernel process helps protect the integrity of disk volumes 39by flushing volatile cached file system data to disk. 40.Pp 41The kernel places all 42.Xr vnode 9 Ns 's 43in a number of queues. 44The 45.Nm 46process works through the queues 47in a round-robin fashion, 48usually processing one queue per second. 49For each 50.Xr vnode 9 51on that queue, 52the 53.Nm 54process forces a write out to disk of its dirty buffers. 55.Pp 56The usual delay between the time buffers are dirtied 57and the time they are synced 58is controlled by the following 59.Xr sysctl 8 60tunable variables: 61.Bl -column "filedelayXXXX" "DefaultXX" "DescriptionXX" 62.It Em Variable Ta Em Default Ta Em Description 63.It Va kern.filedelay Ta 30 Ta "time to delay syncing files" 64.It Va kern.dirdelay Ta 29 Ta "time to delay syncing directories" 65.It Va kern.metadelay Ta 28 Ta "time to delay syncing metadata" 66.El 67.Sh SEE ALSO 68.Xr sync 2 , 69.Xr fsck 8 , 70.Xr sync 8 , 71.Xr sysctl 8 72.Sh HISTORY 73The 74.Nm 75process is a descendant of the 76.Sq update 77command, which appeared in 78.At v6 , 79and was usually started by 80.Pa /etc/rc 81when the system went multi-user. 82A kernel initiated 83.Sq update 84process first appeared in 85.Fx 2.0 . 86.Sh BUGS 87It is possible on some systems that a 88.Xr sync 2 89occurring simultaneously with a crash may cause 90file system damage. 91See 92.Xr fsck 8 . 93