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.\"     From: @(#)swapon.8	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd October 8, 2014
.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.
.Pp
Since
.Nm
cannot be used during kernel initialization, the
.Va dumpdev
variable of
.Xr loader 8
must be used to enable dumps for system panics which occur
during kernel initialization.
.Sh FILES
.Bl -tag -width "/dev/{ada,da}?s?b" -compact
.It Pa /dev/{ada,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 loader 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.