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Kexec/Kdump HOWTO
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Introduction
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Kexec and kdump are new features in the 2.6 mainstream kernel. These features
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are included in Red Hat Enterprise Linux 5. The purpose of these features
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is to ensure faster boot up and creation of reliable kernel vmcores for
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diagnostic purposes.
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Overview
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Kexec
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Kexec is a fastboot mechanism which allows booting a Linux kernel from the
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context of already running kernel without going through BIOS. BIOS can be very
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time consuming especially on the big servers with lots of peripherals. This can
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save a lot of time for developers who end up booting a machine numerous times.
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Kdump
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Kdump is a new kernel crash dumping mechanism and is very reliable because
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the crash dump is captured from the context of a freshly booted kernel and
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not from the context of the crashed kernel. Kdump uses kexec to boot into
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a second kernel whenever system crashes. This second kernel, often called
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a capture kernel, boots with very little memory and captures the dump image.
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The first kernel reserves a section of memory that the second kernel uses
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to boot. Kexec enables booting the capture kernel without going through BIOS
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hence contents of first kernel's memory are preserved, which is essentially
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the kernel crash dump.
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Kdump is supported on the i686, x86_64, ia64 and ppc64 platforms. The
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standard kernel and capture kernel are one in the same on i686, x86_64,
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ia64 and ppc64.
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If you're reading this document, you should already have kexec-tools
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installed. If not, you install it via the following command:
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    # yum install kexec-tools
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Now load a kernel with kexec:
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    # kver=`uname -r` # kexec -l /boot/vmlinuz-$kver
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    --initrd=/boot/initrd-$kver.img \
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        --command-line="`cat /proc/cmdline`"
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NOTE: The above will boot you back into the kernel you're currently running,
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if you want to load a different kernel, substitute it in place of `uname -r`.
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Now reboot your system, taking note that it should bypass the BIOS:
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    # reboot
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How to configure kdump:
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Again, we assume if you're reading this document, you should already have
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kexec-tools installed. If not, you install it via the following command:
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    # yum install kexec-tools
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To be able to do much of anything interesting in the way of debug analysis,
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you'll also need to install the kernel-debuginfo package, of the same arch
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as your running kernel, and the crash utility:
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    # yum --enablerepo=\*debuginfo install kernel-debuginfo.$(uname -m) crash
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Next up, we need to modify some boot parameters to reserve a chunk of memory for
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the capture kernel. With the help of grubby, it's very easy to append
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"crashkernel=128M" to the end of your kernel boot parameters. Note that the X
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values are such that X = the amount of memory to reserve for the capture kernel.
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And based on arch and system configuration, one might require more than 128M to
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be reserved for kdump. One need to experiment and test kdump, if 128M is not
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sufficient, try reserving more memory.
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   # grubby --args="crashkernel=128M" --update-kernel=/boot/vmlinuz-`uname -r`
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Note that there is an alternative form in which to specify a crashkernel
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memory reservation, in the event that more control is needed over the size and
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placement of the reserved memory.  The format is:
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crashkernel=range1:size1[,range2:size2,...][@offset]
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Where range<n> specifies a range of values that are matched against the amount
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of physical RAM present in the system, and the corresponding size<n> value
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specifies the amount of kexec memory to reserve.  For example:
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crashkernel=512M-2G:64M,2G-:128M
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This line tells kexec to reserve 64M of ram if the system contains between
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512M and 2G of physical memory.  If the system contains 2G or more of physical
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memory, 128M should be reserved.
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After making said changes, reboot your system, so that the X MB of memory is
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left untouched by the normal system, reserved for the capture kernel. Take note
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that the output of 'free -m' will show X MB less memory than without this
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parameter, which is expected. You may be able to get by with less than 128M, but
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testing with only 64M has proven unreliable of late. On ia64, as much as 512M
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may be required.
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Now that you've got that reserved memory region set up, you want to turn on
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the kdump init script:
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    # chkconfig kdump on
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Then, start up kdump as well:
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    # systemctl start kdump.service
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This should load your kernel-kdump image via kexec, leaving the system ready
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to capture a vmcore upon crashing. To test this out, you can force-crash
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your system by echo'ing a c into /proc/sysrq-trigger:
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    # echo c > /proc/sysrq-trigger
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You should see some panic output, followed by the system restarting into
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the kdump kernel. When the boot process gets to the point where it starts
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the kdump service, your vmcore should be copied out to disk (by default,
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in /var/crash/<YYYY-MM-DD-HH:MM>/vmcore), then the system rebooted back into
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your normal kernel.
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Once back to your normal kernel, you can use the previously installed crash
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kernel in conjunction with the previously installed kernel-debuginfo to
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perform postmortem analysis:
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    # crash /usr/lib/debug/lib/modules/2.6.17-1.2621.el5/vmlinux
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    /var/crash/2006-08-23-15:34/vmcore
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    crash> bt
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and so on...
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Saving vmcore-dmesg.txt
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----------------------
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Kernel log bufferes are one of the most important information available
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in vmcore. Now before saving vmcore, kernel log bufferes are extracted
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from /proc/vmcore and saved into a file vmcore-dmesg.txt. After
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vmcore-dmesg.txt, vmcore is saved. Destination disk and directory for
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vmcore-dmesg.txt is same as vmcore. Note that kernel log buffers will
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not be available if dump target is raw device.
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Dump Triggering methods:
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This section talks about the various ways, other than a Kernel Panic, in which
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Kdump can be triggered. The following methods assume that Kdump is configured
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on your system, with the scripts enabled as described in the section above.
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1) AltSysRq C
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Kdump can be triggered with the combination of the 'Alt','SysRq' and 'C'
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keyboard keys. Please refer to the following link for more details:
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http://kbase.redhat.com/faq/FAQ_43_5559.shtm
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In addition, on PowerPC boxes, Kdump can also be triggered via Hardware
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Management Console(HMC) using 'Ctrl', 'O' and 'C' keyboard keys.
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2) NMI_WATCHDOG
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In case a machine has a hard hang, it is quite possible that it does not
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respond to keyboard interrupts. As a result 'Alt-SysRq' keys will not help
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trigger a dump. In such scenarios Nmi Watchdog feature can prove to be useful.
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The following link has more details on configuring Nmi watchdog option.
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http://kbase.redhat.com/faq/FAQ_85_9129.shtm
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Once this feature has been enabled in the kernel, any lockups will result in an
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OOPs message to be generated, followed by Kdump being triggered.
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3) Kernel OOPs
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If we want to generate a dump everytime the Kernel OOPses, we can achieve this
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by setting the 'Panic On OOPs' option as follows:
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    # echo 1 > /proc/sys/kernel/panic_on_oops
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This is enabled by default on RHEL5.
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4) NMI(Non maskable interrupt) button
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In cases where the system is in a hung state, and is not accepting keyboard
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interrupts, using NMI button for triggering Kdump can be very useful. NMI
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button is present on most of the newer x86 and x86_64 machines. Please refer
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to the User guides/manuals to locate the button, though in most occasions it
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is not very well documented. In most cases it is hidden behind a small hole
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on the front or back panel of the machine. You could use a toothpick or some
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other non-conducting probe to press the button.
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For example, on the IBM X series 366 machine, the NMI button is located behind
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a small hole on the bottom center of the rear panel.
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To enable this method of dump triggering using NMI button, you will need to set
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the 'unknown_nmi_panic' option as follows:
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   # echo 1 > /proc/sys/kernel/unknown_nmi_panic
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5) PowerPC specific methods:
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On IBM PowerPC machines, issuing a soft reset invokes the XMON debugger(if
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XMON is configured). To configure XMON one needs to compile the kernel with
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the CONFIG_XMON and CONFIG_XMON_DEFAULT options, or by compiling with
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CONFIG_XMON and booting the kernel with xmon=on option.
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Following are the ways to remotely issue a soft reset on PowerPC boxes, which
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would drop you to XMON. Pressing a 'X' (capital alphabet X) followed by an
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'Enter' here will trigger the dump.
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5.1) HMC
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Hardware Management Console(HMC) available on Power4 and Power5 machines allow
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partitions to be reset remotely. This is specially useful in hang situations
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where the system is not accepting any keyboard inputs.
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Once you have HMC configured, the following steps will enable you to trigger
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Kdump via a soft reset:
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On Power4
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  Using GUI
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    * In the right pane, right click on the partition you wish to dump.
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    * Select "Operating System->Reset".
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    * Select "Soft Reset".
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    * Select "Yes".
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  Using HMC Commandline
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    # reset_partition -m <machine> -p <partition> -t soft
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On Power5
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  Using GUI
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    * In the right pane, right click on the partition you wish to dump.
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    * Select "Restart Partition".
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    * Select "Dump".
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    * Select "OK".
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  Using HMC Commandline
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    # chsysstate -m <managed system name> -n <lpar name> -o dumprestart -r lpar
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5.2) Blade Management Console for Blade Center
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To initiate a dump operation, go to Power/Restart option under "Blade Tasks" in
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the Blade Management Console. Select the corresponding blade for which you want
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to initate the dump and then click "Restart blade with NMI". This issues a
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system reset and invokes xmon debugger.
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Advanced Setups:
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In addition to being able to capture a vmcore to your system's local file
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system, kdump can be configured to capture a vmcore to a number of other
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locations, including a raw disk partition, a dedicated file system, an NFS
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mounted file system, or a remote system via ssh/scp. Additional options
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exist for specifying the relative path under which the dump is captured,
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what to do if the capture fails, and for compressing and filtering the dump
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(so as to produce smaller, more manageable, vmcore files).
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In theory, dumping to a location other than the local file system should be
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safer than kdump's default setup, as its possible the default setup will try
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dumping to a file system that has become corrupted. The raw disk partition and
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dedicated file system options allow you to still dump to the local system,
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but without having to remount your possibly corrupted file system(s),
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thereby decreasing the chance a vmcore won't be captured. Dumping to an
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NFS server or remote system via ssh/scp also has this advantage, as well
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as allowing for the centralization of vmcore files, should you have several
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systems from which you'd like to obtain vmcore files. Of course, note that
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these configurations could present problems if your network is unreliable.
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Advanced setups are configured via modifications to /etc/kdump.conf,
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which out of the box, is fairly well documented itself. Any alterations to
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/etc/kdump.conf should be followed by a restart of the kdump service, so
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the changes can be incorporated in the kdump initrd. Restarting the kdump
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service is as simple as '/sbin/systemctl restart kdump.service'.
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Note that kdump.conf is used as a configuration mechanism for capturing dump
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files from the initramfs (in the interests of safety), the root file system is
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mounted, and the init process is started, only as a last resort if the
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initramfs fails to capture the vmcore.  As such, configuration made in
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/etc/kdump.conf is only applicable to capture recorded in the initramfs.  If
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for any reason the init process is started on the root file system, only a
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simple copying of the vmcore from /proc/vmcore to /var/crash/$DATE/vmcore will
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be preformed.
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For both local filesystem and nfs dump the dump target must be mounted before
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building kdump initramfs. That means one needs to put an entry for the dump
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file system in /etc/fstab so that after reboot when kdump service starts,
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it can find the dump target and build initramfs instead of failing.
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Usually the dump target should be used only for kdump. If you worry about
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someone uses the filesystem for something else other than dumping vmcore
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you can mount it as read-only. Mkdumprd will still remount it as read-write
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for creating dump directory and will move it back to read-only afterwards.
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Raw partition
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Raw partition dumping requires that a disk partition in the system, at least
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as large as the amount of memory in the system, be left unformatted. Assuming
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/dev/vg/lv_kdump is left unformatted, kdump.conf can be configured with
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'raw /dev/vg/lv_kdump', and the vmcore file will be copied via dd directly
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onto partition /dev/vg/lv_kdump. Restart the kdump service via
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'/sbin/systemctl restart kdump.service' to commit this change to your kdump
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initrd. Dump target should be persistent device name, such as lvm or device
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mapper canonical name.
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Dedicated file system
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Similar to raw partition dumping, you can format a partition with the file
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system of your choice, Again, it should be at least as large as the amount
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of memory in the system. Assuming it should be at least as large as the
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amount of memory in the system. Assuming /dev/vg/lv_kdump has been
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formatted ext4, specify 'ext4 /dev/vg/lv_kdump' in kdump.conf, and a
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vmcore file will be copied onto the file system after it has been mounted.
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Dumping to a dedicated partition has the advantage that you can dump multiple
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vmcores to the file system, space permitting, without overwriting previous ones,
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as would be the case in a raw partition setup. Restart the kdump service via
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'/sbin/systemctl restart kdump.service' to commit this change to
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your kdump initrd.  Note that for local file systems ext4 and ext2 are
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supported as dumpable targets.  Kdump will not prevent you from specifying
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other filesystems, and they will most likely work, but their operation
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cannot be guaranteed.  for instance specifying a vfat filesystem or msdos
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filesystem will result in a successful load of the kdump service, but during
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crash recovery, the dump will fail if the system has more than 2GB of memory
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(since vfat and msdos filesystems do not support more than 2GB files).
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Be careful of your filesystem selection when using this target.
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It is recommended to use persistent device names or UUID/LABEL for file system
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dumps. One example of persistent device is /dev/vg/<devname>.
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NFS mount
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Dumping over NFS requires an NFS server configured to export a file system
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with full read/write access for the root user. All operations done within
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the kdump initial ramdisk are done as root, and to write out a vmcore file,
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we obviously must be able to write to the NFS mount. Configuring an NFS
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server is outside the scope of this document, but either the no_root_squash
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or anonuid options on the NFS server side are likely of interest to permit
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the kdump initrd operations write to the NFS mount as root.
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Assuming your're exporting /dump on the machine nfs-server.example.com,
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once the mount is properly configured, specify it in kdump.conf, via
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'nfs nfs-server.example.com:/dump'. The server portion can be specified either
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by host name or IP address. Following a system crash, the kdump initrd will
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mount the NFS mount and copy out the vmcore to your NFS server. Restart the
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kdump service via '/sbin/systemctl restart kdump.service' to commit this change
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to your kdump initrd.
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Remote system via ssh/scp
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Dumping over ssh/scp requires setting up passwordless ssh keys for every
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machine you wish to have dump via this method. First up, configure kdump.conf
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for ssh/scp dumping, adding a config line of 'ssh user@server', where 'user'
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can be any user on the target system you choose, and 'server' is the host
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name or IP address of the target system. Using a dedicated, restricted user
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account on the target system is recommended, as there will be keyless ssh
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access to this account.
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Once kdump.conf is appropriately configured, issue the command
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'kdumpctl propagate' to automatically set up the ssh host keys and transmit
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the necessary bits to the target server. You'll have to type in 'yes'
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to accept the host key for your targer server if this is the first time
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you've connected to it, and then input the target system user's password
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to send over the necessary ssh key file. Restart the kdump service via
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'/sbin/systemctl restart kdump.service' to commit this change to your kdump initrd.
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Path
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====
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"path" represents the file system path in which vmcore will be saved. In
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fact kdump creates a directory $hostip-$date with-in "path" and saves
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vmcore there. So practically dump is saved in $path/$hostip-$date/. To
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simplify discussion further, if we say dump will be saved in $path, it
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is implied that kdump will create another directory inside path and
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save vmcore there.
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If a dump target is specified in kdump.conf, then "path" is relative to the
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specified dump target. For example, if dump target is "ext4 /dev/sda", then
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dump will be saved in "$path" directory on /dev/sda.
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Same is the case for nfs dump. If user specified "nfs foo.com:/export/tmp/"
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as dump target, then dump will effectively be saved in
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"foo.com:/export/tmp/var/crash/" directory.
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Interpretation of path changes a bit if user has not specified a dump
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target explicitly in kdump.conf. In this case, "path" represents the
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absolute path from root. And dump target and adjusted path are arrived
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at automatically depending on what's mounted in the current system.
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Following are few examples.
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path /var/crash/
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----------------
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Assuming there is no disk mounted on /var/ or on /var/crash, dump will
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be saved on disk backing rootfs in directory /var/crash.
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path /var/crash/ (A separate disk mounted on /var)
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--------------------------------------------------
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Say a disk /dev/sdb is mouted on /var. In this case dump target will
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become /dev/sdb and path will become "/crash" and dump will be saved
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on "sdb:/crash/" directory.
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path /var/crash/ (NFS mounted on /var)
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-------------------------------------
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Say foo.com:/export/tmp is mounted on /var. In this case dump target is
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nfs server and path will be adjusted to "/crash" and dump will be saved to
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foo.com:/export/tmp/crash/ directory.
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Kdump boot directory
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====================
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Usually kdump kernel is the same as 1st kernel. So kdump will try to find
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kdump kernel under /boot according to /proc/cmdline. E.g we execute below
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command and get an output:
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	cat /proc/cmdline
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	BOOT_IMAGE=/xxx/vmlinuz-3.yyy.zzz  root=xxxx .....
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Then kdump kernel will be /boot/xxx/vmlinuz-3.yyy.zzz.
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However a variable KDUMP_BOOTDIR in /etc/sysconfig/kdump is provided to
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user if kdump kernel is put in a different directory.
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Kdump Post-Capture Executable
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It is possible to specify a custom script or binary you wish to run following
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an attempt to capture a vmcore. The executable is passed an exit code from
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the capture process, which can be used to trigger different actions from
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within your post-capture executable.
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Kdump Pre-Capture Executable
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It is possible to specify a custom script or binary you wish to run before
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capturing a vmcore. Exit status of this binary is interpreted:
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0 - continue with dump process as usual
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non 0 - reboot the system
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Extra Binaries
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If you have specific binaries or scripts you want to have made available
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within your kdump initrd, you can specify them by their full path, and they
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will be included in your kdump initrd, along with all dependent libraries.
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This may be particularly useful for those running post-capture scripts that
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rely on other binaries.
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Extra Modules
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By default, only the bare minimum of kernel modules will be included in your
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kdump initrd. Should you wish to capture your vmcore files to a non-boot-path
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storage device, such as an iscsi target disk or clustered file system, you may
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need to manually specify additional kernel modules to load into your kdump
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initrd.
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Default action
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==============
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Default action specifies what to do when dump to configured dump target
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fails. By default, default action is "reboot" and that is system reboots
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if attempt to save dump to dump target fails.
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There are other default actions available though.
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- dump_to_rootfs
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	This option tries to mount root and save dump on root filesystem
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	in a path specified by "path". This option will generally make
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	sense when dump target is not root filesystem. For example, if
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	dump is being saved over network using "ssh" then one can specify
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	default to "dump_to_rootfs" to try saving dump to root filesystem
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	if dump over network fails.
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- shell
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	Drop into a shell session inside initramfs.
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- halt
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	Halt system after failure
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- poweroff
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	Poweroff system after failure.
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Compression and filtering
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The 'core_collector' parameter in kdump.conf allows you to specify a custom
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dump capture method. The most common alternate method is makedumpfile, which
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is a dump filtering and compression utility provided with kexec-tools. On
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some architectures, it can drastically reduce the size of your vmcore files,
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which becomes very useful on systems with large amounts of memory.
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A typical setup is 'core_collector makedumpfile -F -l --message-level 1 -d 31',
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but check the output of '/sbin/makedumpfile --help' for a list of all available
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options (-i and -g don't need to be specified, they're automatically taken care
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of). Note that use of makedumpfile requires that the kernel-debuginfo package
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corresponding with your running kernel be installed.
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Core collector command format depends on dump target type. Typically for
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filesystem (local/remote), core_collector should accept two arguments.
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First one is source file and second one is target file. For ex.
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ex1.
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---
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core_collector "cp --sparse=always"
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Above will effectively be translated to:
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cp --sparse=always /proc/vmcore <dest-path>/vmcore
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ex2.
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---
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core_collector "makedumpfile -l --message-level 1 -d 31"
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Above will effectively be translated to:
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makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dest-path>/vmcore
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For dump targets like raw and ssh, in general, core collector should expect
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one argument (source file) and should output the processed core on standard
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output (There is one exception of "scp", discussed later). This standard
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output will be saved to destination using appropriate commands.
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raw dumps core_collector examples:
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---------
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ex3.
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---
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core_collector "cat"
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Above will effectively be translated to.
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cat /proc/vmcore | dd of=<target-device>
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ex4.
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---
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core_collector "makedumpfile -F -l --message-level 1 -d 31"
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Above will effectively be translated to.
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makedumpfile -F -l --message-level 1 -d 31 | dd of=<target-device>
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ssh dumps core_collector examples:
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---------
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ex5.
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---
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core_collector "cat"
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Above will effectively be translated to.
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cat /proc/vmcore | ssh <options> <remote-location> "dd of=path/vmcore"
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ex6.
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---
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core_collector "makedumpfile -F -l --message-level 1 -d 31"
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Above will effectively be translated to.
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makedumpfile -F -l --message-level 1 -d 31 | ssh <options> <remote-location> "dd of=path/vmcore"
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There is one exception to standard output rule for ssh dumps. And that is
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scp. As scp can handle ssh destinations for file transfers, one can
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specify "scp" as core collector for ssh targets (no output on stdout).
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ex7.
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----
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core_collector "scp"
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Above will effectively be translated to.
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scp /proc/vmcore <user@host>:path/vmcore
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About default core collector
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----------------------------
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Default core_collector for ssh/raw dump is:
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"makedumpfile -F -l --message-level 1 -d 31".
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Default core_collector for other targets is:
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"makedumpfile -l --message-level 1 -d 31".
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Even if core_collector option is commented out in kdump.conf, makedumpfile
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is default core collector and kdump uses it internally.
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If one does not want makedumpfile as default core_collector, then they
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need to specify one using core_collector option to change the behavior.
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Note: If "makedumpfile -F" is used then you will get a flattened format
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vmcore.flat, you will need to use "makedumpfile -R" to rearrange the
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dump data from stdard input to a normal dumpfile (readable with analysis
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tools).
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For example: "makedumpfile -R vmcore < vmcore.flat"
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Caveats:
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Console frame-buffers and X are not properly supported. If you typically run
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with something along the lines of "vga=791" in your kernel config line or
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have X running, console video will be garbled when a kernel is booted via
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kexec. Note that the kdump kernel should still be able to create a dump,
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and when the system reboots, video should be restored to normal.
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Notes on resetting video:
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Video is a notoriously difficult issue with kexec.  Video cards contain ROM code
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that controls their initial configuration and setup.  This code is nominally
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accessed and executed from the Bios, and otherwise not safely executable. Since
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the purpose of kexec is to reboot the system without re-executing the Bios, it
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is rather difficult if not impossible to reset video cards with kexec.  The
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result is, that if a system crashes while running in a graphical mode (i.e.
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running X), the screen may appear to become 'frozen' while the dump capture is
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taking place.  A serial console will of course reveal that the system is
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operating and capturing a vmcore image, but a casual observer will see the
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system as hung until the dump completes and a true reboot is executed.
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There are two possiblilties to work around this issue.  One is by adding
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--reset-vga to the kexec command line options in /etc/sysconfig/kdump.  This
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tells kdump to write some reasonable default values to the video card register
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file, in the hopes of returning it to a text mode such that boot messages are
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visible on the screen.  It does not work with all video cards however.
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Secondly, it may be worth trying to add vga15fb.ko to the extra_modules list in
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/etc/kdump.conf.  This will attempt to use the video card in framebuffer mode,
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which can blank the screen prior to the start of a dump capture.
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Notes on rootfs mount:
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Dracut is designed to mount rootfs by default. If rootfs mounting fails it
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will refuse to go on. So kdump leaves rootfs mounting to dracut currently.
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We make the assumtion that proper root= cmdline is being passed to dracut
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initramfs for the time being. If you need modify "KDUMP_COMMANDLINE=" in
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/etc/sysconfig/kdump, you will need to make sure that appropriate root=
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options are copied from /proc/cmdline. In general it is best to append
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command line options using "KDUMP_COMMANDLINE_APPEND=" instead of replacing
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the original command line completely.
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Debugging Tips
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--------------
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- One can drop into a shell before/after saving vmcore with the help of
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  using kdump_pre/kdump_post hooks. Use following in one of the pre/post
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  scripts to drop into a shell.
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  #!/bin/bash
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  _ctty=/dev/ttyS0
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  setsid /bin/sh -i -l 0<>$_ctty 1<>$_ctty 2<>$_ctty
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  One might have to change the terminal depending on what they are using.
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- Serial console logging for virtual machines
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  I generally use "virsh console <domain-name>" to get to serial console.
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  I noticed after dump saving system reboots and when grub menu shows up
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  some of the previously logged messages are no more there. That means
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  any important debugging info at the end will be lost.
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  One can log serial console as follows to make sure messages are not lost.
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  virsh ttyconsole <domain-name>
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  ln -s <name-of-tty> /dev/modem
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  minicom -C /tmp/console-logs
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  Now minicom should be logging serial console in file console-logs.
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