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.\"     From: @(#)swapon.8	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd May 12, 1995
.Dt DUMPON 8
.Os FreeBSD
.Sh NAME
.Nm dumpon
.Nd "specify a device for crash dumps"
.Sh SYNOPSIS
.Nm
.Op Fl v
.Ar special_file
.Nm
.Op Fl v
.Ar off
.Sh DESCRIPTION
.Nm Dumpon
is used to specify a device where the kernel can save a crash dump in
the case of a panic.
.Pp
Calls to 
.Nm
normally occur from the system multi-user initialization file
.Pa /etc/rc ,
controlled by the
.Dq dumpdev
variable in the boot time configuration file 
.Pa /etc/rc.conf .
.Pp
The size of the specified dump device must be at least 64 KB greater the 
size of physical memory.
.Pp
The
.Fl v
flag causes 
.Nm
to be verbose about its activity.
.Sh NOTES
Since a
.Xr panic 9
condition may occur in a situation
where the kernel cannot trust its internal representation
of the state of any given filesystem,
one of the system swap devices,
and
.Em not
a device containing a filesystem,
should be used as the dump device.
.Pp
The 
.Nm
program operates by setting the
.Xr sysctl 3
MIB variable
.Dq kern.dumpdev
to the device number of the designated 
.Ar special_file
or to
.Dv NODEV
(meaning that no dumps are to be taken) if
.Ar special_file
is the text string:
.Dq Li off .
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr fstab 5 ,
.Xr rc.conf 5 ,
.Xr config 8 ,
.Xr init 8 ,
.Xr rc 8 ,
.Xr savecore 8 ,
.Xr swapon 8 ,
.Xr panic 9
.Sh FILES
.Bl -tag -width "/dev/{ad,da}?s?b" -compact
.It Pa /dev/{ad,da}?s?b
standard swap areas
.It Pa /etc/rc.conf
boot-time system configuration
.El
.Sh BUGS
Because the filesystem layer is already dead by the time a crash dump
is taken, it is not possible to send crash dumps directly to a file.
.Pp
Since
.Nm
can only be called after the system enters single-user mode,
it can not be used to create dumps
for system panics during kernel initialization.
.Sh HISTORY
The
.Nm
command appeared in
.Fx 2.1 .