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.\"     From: @(#)swapon.8	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd May 12, 1995
.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
.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
For most systems 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.
.Pp
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 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.