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.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     From: @(#)swapon.8	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd April 29, 2013
.Dt DUMPON 8
.Os
.Sh NAME
.Nm dumpon
.Nd "specify a device for crash dumps"
.Sh SYNOPSIS
.Nm
.Op Fl v
.Ar special_file
.Nm
.Op Fl v
.Cm off
.Nm
.Op Fl v
.Fl l
.Sh DESCRIPTION
The
.Nm
utility 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 default type of kernel crash dump is the mini crash dump.
Mini crash dumps hold only memory pages in use by the kernel.
Alternatively, full memory dumps can be enabled by setting the
.Va debug.minidump
.Xr sysctl 8
variable to 0.
.Pp
For systems using full memory dumps, the size of the specified dump
device must be at
least the size of physical memory.
Even though an additional 64 kB header is added to the dump, the BIOS for a
platform typically holds back some memory, so it is not usually
necessary to size the dump device larger than the actual amount of RAM
available in the machine.
Also, when using full memory dumps, the
.Nm
utility will refuse to enable a dump device which is smaller than the
total amount of physical memory as reported by the
.Va hw.physmem
.Xr sysctl 8
variable.
.Pp
The
.Fl l
flag causes
.Nm
to print the current dump device or _PATH_DEVNULL ("/dev/null") if no device is
configured.
.Pp
The
.Fl v
flag causes
.Nm
to be verbose about its activity.
.Sh IMPLEMENTATION 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 file system,
one of the system swap devices,
and
.Em not
a device containing a file system,
should be used as the dump device.
.Pp
The
.Nm
utility operates by opening
.Ar special_file
and making a
.Dv DIOCSKERNELDUMP
.Xr ioctl 2
request on it to save kernel crash dumps.
If
.Ar special_file
is the text string:
.Dq Li off ,
.Nm
performs a
.Dv DIOCSKERNELDUMP
.Xr ioctl 2
on
.Pa /dev/null
and thus instructs the kernel not to save crash dumps.
.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 SEE ALSO
.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 HISTORY
The
.Nm
utility appeared in
.Fx 2.1 .
.Sh BUGS
Because the file system layer is already dead by the time a crash dump
is taken, it is not possible to send crash dumps directly to a file.