Command Section
DUMPON(8)               FreeBSD System Manager's Manual              DUMPON(8)

     dumpon - specify a device for crash dumps

     dumpon [-v] special_file
     dumpon [-v] off
     dumpon [-v] -l

     The dumpon utility is used to specify a device where the kernel can save
     a crash dump in the case of a panic.

     Calls to dumpon normally occur from the system multi-user initialization
     file /etc/rc, controlled by the ``dumpdev'' variable in the boot time
     configuration file /etc/rc.conf.

     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 debug.minidump sysctl(8)
     variable to 0.

     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 dumpon utility will
     refuse to enable a dump device which is smaller than the total amount of
     physical memory as reported by the hw.physmem sysctl(8) variable.

     The -l flag causes dumpon to print the current dump device or
     _PATH_DEVNULL ("/dev/null") if no device is configured.

     The -v flag causes dumpon to be verbose about its activity.

     Since a 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 not a device containing a
     file system, should be used as the dump device.

     The dumpon utility operates by opening special_file and making a
     DIOCSKERNELDUMP ioctl(2) request on it to save kernel crash dumps.  If
     special_file is the text string: ``off'', dumpon performs a
     DIOCSKERNELDUMP ioctl(2) on /dev/null and thus instructs the kernel not
     to save crash dumps.

     Since dumpon cannot be used during kernel initialization, the dumpdev
     variable of loader(8) must be used to enable dumps for system panics
     which occur during kernel initialization.

     /dev/{ada,da}?s?b  standard swap areas
     /etc/rc.conf       boot-time system configuration

     fstab(5), rc.conf(5), config(8), init(8), loader(8), rc(8), savecore(8),
     swapon(8), panic(9)

     The dumpon utility appeared in FreeBSD 2.0.5.

     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.

FreeBSD 11.1-RELEASE-p4         October 3, 2016        FreeBSD 11.1-RELEASE-p4
Command Section