This section explains a bit about system crashes
and (very briefly) how to analyze crash dumps.
When the system crashes voluntarily it prints a message of the form
"panic: why i gave up the ghost"
on the console, and if dumps have been enabled (see
dumpon(8)),
takes a dump on a mass storage peripheral,
and then invokes an automatic reboot procedure as
described in
reboot(8).
Unless some unexpected inconsistency is encountered in the state
of the file systems due to hardware or software failure, the system
will then resume multi-user operations.
The system has a large number of internal consistency checks; if one
of these fails, then it will panic with a very short message indicating
which one failed.
In many instances, this will be the name of the routine which detected
the error, or a two-word description of the inconsistency.
A full understanding of most panic messages requires perusal of the
source code for the system.
The most common cause of system failures is hardware failure, which
can reflect itself in different ways.
Here are the messages which
are most likely, with some hints as to causes.
Left unstated in all cases is the possibility that hardware or software
error produced the message in some unexpected way.
"cannot mount root"
This panic message results from a failure to mount the root file system
during the bootstrap process.
Either the root file system has been corrupted,
or the system is attempting to use the wrong device as root file system.
Usually, an alternate copy of the system binary or an alternate root
file system can be used to bring up the system to investigate.
Most often
this is done by the use of the boot floppy you used to install the system,
and then using the
``fixit''
floppy.
"init: not found"
This is not a panic message, as reboots are likely to be futile.
Late in the bootstrap procedure, the system was unable to locate
and execute the initialization process,
init(8).
The root file system is incorrect or has been corrupted, or the mode
or type of
/sbin/init
forbids execution or is totally missing.
"ffs_realloccg: bad optim"
"ffs_valloc: dup alloc"
"ffs_alloccgblk: cyl groups corrupted"
"ffs_alloccg: map corrupted"
"blkfree: freeing free block"
"blkfree: freeing free frag"
"ifree: freeing free inode"
These panic messages are among those that may be produced
when file system inconsistencies are detected.
The problem generally results from a failure to repair damaged file systems
after a crash, hardware failures, or other condition that should not
normally occur.
A file system check will normally correct the problem.
"timeout table full"
This really should not be a panic, but until the data structure
involved is made to be extensible, running out of entries causes a crash.
If this happens, make the timeout table bigger.
"init died (signal #, exit #)"
The system initialization process has exited with the specified
signal number and exit code.
This is bad news, as no new users will then be able to log in.
Rebooting is the only fix, so the
system just does it right away.
That completes the list of panic types you are likely to see.
If the system has been configured to take crash dumps (see
dumpon(8)),
then when it crashes it will write (or at least attempt to write)
an image of memory into the back end of the dump device,
usually the same as the primary swap
area.
After the system is rebooted, the program
savecore(8)
runs and preserves a copy of this core image and the current
system in a specified directory for later perusal.
See
savecore(8)
for details.
To analyze a dump you should begin by running
kgdb(1)
on the system load image and core dump.
If the core image is the result of a panic,
the panic message is printed.
For more details consult the chapter on kernel debugging in
the
"FreeBSD Developers' Handbook"
(http://www.FreeBSD.org/
)
