# Getting a system core dump 
[[!toc levels=3 ]]�  A full system core dump (also called post-mortem dump) is sometimes needed to track down a bug in the DragonFlyBSD kernel.

Please note: A full core dump is obtained by triggering a kernel panic. Like when pulling the power plug from the wall socket, unsaved data will be lost, recently saved data might be lost, and filesystems mounted async might be destroyed.

## Requirements 

* a dump partition at least as large as main memory (usually swap)

* `/var/crash` must be on a filesystem with enough space to hold the dump (the size of main memory).

* sync on panic should be disabled in the kernel

## Setup 
### Setting up the dump device 

* If your swap partition is as large as main memory, it can be used as a dump device.

* If your swap partition is too small, you use any partition that doesn't contain a filesystem.

* Add this to `/etc/rc.conf`:

    
     dumpdev="/dev/<devicenode>"


* Please refer to [dumpon(8)](http://leaf.dragonflybsd.org/cgi/web-man?command#dumpon&section8) (http://leaf.dragonflybsd.org/cgi/web-man?command=dumpon&section=8) for details.

### Setting up /var/crash 
If `/var/crash` is residing on a filesystem without enough room to accommodate the core dump, you can move it to a different filesystem and use a symbolic link.

    
    # cpdup /var/crash /home/var.crash
    # rm -rf /var/crash
    # ln -s /home/var.crash /var/crash

### Disabling sync on panic 
Add this to `/etc/sysctl.conf`

    
    kern.sync_on_panic=0

# Getting a core dump 
Hit `<ctrl-alt-esc>` at the desired moment. This will break into the kernel debugger. Then type:

    
    # panic

The machine should reboot automatically. Sometimes it might require a cold reset.

If the machine has dropped to a dbb> prompt by itself you can type the following to get a useful dump:

    
    # call dumpsys

During the following startup the core dump will be automatically saved to `/var/crash`.

# Getting a coredump on a remote machine
Set debug.debugger_on_panic to 0 or configure your kernel with options DDB_UNATTENDED.

# Useful hints 

* You can have the settings described above take effect without rebooting using the following commands:

    
    # dumpon /dev/<devicenode>
    # sysctl kern.sync_on_panic=0


* Core files can get quite unwieldy with large amounts of main memory. In order to temporarily limit the amount of physical memory add a line similar to the following to /boot/loader.conf (and remove it again once you've produced the core dump):

    
    hw.physmem="64M"


* On systems derived from DEVELOPMENT (HEAD) after 20 August 2005, you can get a panic directly by pressing `<ctrl-alt-shift-esc>` if you set the `sysctl machdep.enable_panic_key to 1`.