DUMPON(8)

NAME
dumpon, dumpoff - specify a device for crash dumps

SYNOPSIS
dumpon special_file
dumpon off
dumpoff

DESCRIPTION
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 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.

NOTES
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 setting the sysctl(3) MIB variable kern.dumpdev to the device number of the designated special_file or to NODEV (meaning that no dumps are to be taken) if the utility is invoked as dumpoff or if special_file is the text string: “off”.

In order to change the device for crash dumps, one needs to run the dumpoff utility to disable the current dump device before configuring the new one.

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.

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

SEE ALSO
sysctl(3), fstab(5), rc.conf(5), init(8), loader(8), rc(8), savecore(8), swapon(8), panic(9)

HISTORY
The dumpon utility appeared in FreeBSD 2.0.5. The dumpoff utility first appeared in DragonFly 5.3.

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.