11-10-2011
A crashed kernel writes no files... This is a safety feature preventing it from corrupting the disk.
Meaning, kernel panics don't get saved to disk, which is often unfortunate.
They usually get dumped to the raw VGA console. If you don't let your system start X11, you'll probably see them. You may need a digital camera or somesuch to capture the message since, as before, a crashed kernel writes no files.
You can also configure them to get sent to a serial port instead, I think, if your system has one, so a different machine or a serial printer can capture them.
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LEARN ABOUT MINIX
textdump
TEXTDUMP(4) BSD Kernel Interfaces Manual TEXTDUMP(4)
NAME
textdump -- textdump kernel dumping facility
SYNOPSIS
options KDB
options DDB
DESCRIPTION
The textdump facility allows the capture of kernel debugging information to disk in a human-readable rather than the machine-readable form
normally used with kernel memory dumps and minidumps. This representation, while less complete in that it does not capture full kernel
state, can provide debugging information in a more compact, portable, and persistent form than a traditional dump. By combining textdump
with other ddb(4) facilities, such as scripting and output capture, detailed bug information can be captured in a fully automated manner.
FORMAT
textdump data is stored in a dump partition in the same style as a regular memory dump, and will be automatically extracted by savecore(8) if
present on boot.
textdump files are stored in the tar(5) format, and consist of one or more text files, each storing a particular type of debugging output.
The following parts may be present:
ddb.txt Captured ddb(4) output, if the capture facility has been used. May be disabled by clearing the debug.ddb.textdump.do_ddb
sysctl.
config.txt Kernel configuration, if has been compiled into the kernel. May be disabled by clearing the debug.ddb.textdump.do_config
sysctl.
msgbuf.txt Kernel message buffer, including recent console output if the capture facility has been used. May be disabled by clearing the
debug.ddb.textdump.do_msgbuf sysctl.
panic.txt Kernel panic string, if the kernel panicked before the dump was generated. May be disabled by clearing the
debug.ddb.textdump.do_panic sysctl.
version.txt Kernel version string. My be disabled by clearing the debug.ddb.textdump.do_version sysctl.
Kernel textdumps may be extracted using tar(1).
CONFIGURATION
The textdump facility is enabled as part of the kernel debugger using options KDB and options DDB. By default, kernel dumps generated on
panic or via explicit requests for a dump will be regular memory dumps; however, by using the textdump set command in ddb(4), or by setting
the debug.ddb.textdump.pending sysctl to 1 using sysctl(8), it is possible to request that the next dump be a textdump.
If at the ddb(4) command line, the commands textdump set, textdump status, and textdump unset may be used to set, query, and clear the
textdump pending flag.
As with regular kernel dumps, a dump partition must be automatically or manually configured using dumpon(8).
EXAMPLES
In the following example, the script kdb.enter.panic will run when the kernel debugger is entered as a result of a panic, enable output cap-
ture, dump several useful pieces of debugging information, and then invoke panic in order to force a kernel dump to be written out followed
by a reboot:
script kdb.enter.panic=textdump set; capture on; show allpcpu; bt;
ps; alltrace; show alllock; call doadump; reset
In the following example, the script kdb.enter.witness will run when the kernel debugger is entered as a result of a witness violation,
printing lock-related information for the user:
script kdb.enter.witness=show locks
These scripts may also be configured using the ddb(8) utility.
SEE ALSO
tar(1), ddb(4), tar(5), ddb(8), dumpon(8), savecore(8), sysctl(8)
HISTORY
The textdump facility first appeared in FreeBSD 7.1.
AUTHORS
The textdump facility was created by Robert N. M. Watson.
BSD
December 24, 2008 BSD