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.
For most systems the size of the specified dump device must be at least the size of physical memory. Even though an additional 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.
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 -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.