.\" Copyright (c) 1996
.\"	Mike Pritchard <mpp@FreeBSD.org>.  All rights reserved.
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.\" $FreeBSD$
.\"
.Dd February 11, 1996
.Dt UPDATE 4
.Os FreeBSD
.Sh NAME
.Nm update
.Nd flush internal filesystem caches to disk periodically
.Sh SYNOPSIS
.Nm update
.Sh DESCRIPTION
The
.Nm update
kernel process helps protect the integrity of disk volumes
by flushing
volatile cached filesystem data
to disk.  This is done at thirty second intervals by default.
The time period may be changed by using
.Xr sysctl 8
to modify the
.Va kern.update
variable.
The 
.Nm update
process does the equivalent of the 
.Xr sync 2
function call to do the task.
.Sh SEE ALSO
.Xr sync 2 ,
.Xr fsck 8 ,
.Xr init 8 ,
.Xr rc 8 ,
.Xr sync 8 ,
.Xr sysctl 8
.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 .
.Sh HISTORY
An
.Nm update
command appeared in
.At v6 ,
and was usually started by
.Pa /etc/rc
when the system went multi-user.
A kernel initiated
.Nm update
process first appeared in
.Fx 2.0 .