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