When running a development kernel (e.g., FreeBSD-CURRENT), such as a kernel under extreme conditions (e.g., very high load averages, tens of thousands of connections, exceedingly high number of concurrent users, hundreds of jail(8)s, etc.), or using a new feature or device driver on FreeBSD-STABLE (e.g., PAE), sometimes a kernel will panic. In the event that it does, this chapter will demonstrate how to extract useful information out of a crash.
A system reboot is inevitable once a kernel panics. Once a system is rebooted, the contents of a system's physical memory (RAM) is lost, as well as any bits that are on the swap device before the panic. To preserve the bits in physical memory, the kernel makes use of the swap device as a temporary place to store the bits that are in RAM across a reboot after a crash. In doing this, when FreeBSD boots after a crash, a kernel image can now be extracted and debugging can take place.
Note: A swap device that has been configured as a dump device still acts as a swap device. Dumps to non-swap devices (such as tapes or CDRWs, for example) are not supported at this time. A “swap device” is synonymous with a “swap partition.”
Several types of kernel crash dumps are available: full memory dumps, which hold the complete contents of physical memory, minidumps, which hold only memory pages in use by the kernel (FreeBSD 6.2 and higher), and textdumps, which hold captured scripted or interactive debugger output (FreeBSD 7.1 and higher). Minidumps are the default dump type as of FreeBSD 7.0, and in most cases will capture all necessary information present in a full memory dump, as most problems can be isolated only using kernel state.
Before the kernel will dump the contents of its physical memory to a dump device, a
dump device must be configured. A dump device is specified by using the dumpon(8) command to
tell the kernel where to save kernel crash dumps. The dumpon(8) program must
be called after the swap partition has been configured with swapon(8). This is
normally handled by setting the dumpdev variable in rc.conf(5) to the path
of the swap device (the recommended way to extract a kernel dump) or AUTO to use the first configured swap device. AUTO is the default as of FreeBSD 6.0.
Tip: Check /etc/fstab or swapinfo(8) for a list of swap devices.
Important: Make sure the
dumpdirspecified in rc.conf(5) exists before a kernel crash!# mkdir /var/crash # chmod 700 /var/crashAlso, remember that the contents of /var/crash is sensitive and very likely contains confidential information such as passwords.
Once a dump has been written to a dump device, the dump must be extracted before the
swap device is mounted. To extract a dump from a dump device, use the savecore(8) program.
If dumpdev has been set in rc.conf(5), savecore(8) will be
called automatically on the first multi-user boot after the crash and before the swap
device is mounted. The location of the extracted core is placed in the rc.conf(5) value dumpdir, by default /var/crash and will
be named vmcore.0.
In the event that there is already a file called vmcore.0 in
/var/crash (or whatever dumpdir is
set to), the kernel will increment the trailing number for every crash to avoid
overwriting an existing vmcore (e.g., vmcore.1). While debugging, it is highly likely that you will want
to use the highest version vmcore in /var/crash when searching for the right vmcore.
Tip: If you are testing a new kernel but need to boot a different one in order to get your system up and running again, boot it only into single user mode using the
-sflag at the boot prompt, and then perform the following steps:# fsck -p # mount -a -t ufs # make sure /var/crash is writable # savecore /var/crash /dev/ad0s1b # exit # exit to multi-userThis instructs savecore(8) to extract a kernel dump from /dev/ad0s1b and place the contents in /var/crash. Do not forget to make sure the destination directory /var/crash has enough space for the dump. Also, do not forget to specify the correct path to your swap device as it is likely different than /dev/ad0s1b!