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Stefan Lasiewski
Asked: 2014-06-07 13:46:41 +0800 CST

Why is Linux kdump not writing to /var/crash?

  • 772

It's happened again! I have 4 servers which are crashing periodically, and there is no information printed to the system logs or the serial console.

In addition, the Linux kdump service isn't writing core dumps to the default location of /var/crash.

  • Can you help me figure out why?
  • Does it matter if my root filesystem is a LVM volume?

Here is what I've tried.

  1. My system is Scientific Linux 6.5 with the latest kernel.

    [root@host1 ~]# uname -r
2.6.32-431.11.2.el6.x86_64
[root@host1 ~]# cat /etc/issue
Scientific Linux release 6.5 (Carbon)

  2. The file /etc/kdump.conf is the vanilla file containing the default settings. Most lines are commented out, there are only two active lines for path and core_collector.

    #net my.server.com:/export/tmp
#net [email protected]
path /var/crash
core_collector makedumpfile -c --message-level 1 -d 31
#core_collector scp

  3. I ensure that the kdump service is running, and that kdump doesn't need to rebuild my initrd.

    [root@host1 ~]# chkconfig --list kdump
kdump           0:off   1:off   2:off   3:on    4:on    5:on    6:off
[root@host1 ~]# /etc/init.d/kdump restart
Stopping kdump:                                            [  OK  ]
Starting kdump:                                            [  OK  ]
[root@host1 ~]#

  4. Then, I force a Kernel crash using these commands borrowed from the RHEL6 Deployment Guide: Chapter 29. The kdump Crash Recovery Service:

    Then type the following commands at a shell prompt:

    echo 1 > /proc/sys/kernel/sysrq
echo c > /proc/sysrq-trigger

    This will force the Linux kernel to crash

  5. The system crashes. I can view the progress on my serial console. I see the message Saving to the local filesystem UUID=e7abcdeb-1987-4c69-a867-fabdceffghi2, but immediately after that I see the strange message of Usage: fsck.ext4, which sort of looks like something is accidentally calling fsck instead of whatever it should be doing. I see no mention of an out-of-memory error or anything.

    host1.example.org login: SysRq : Trigger a crash
BUG: unable to handle kernel NULL pointer dereference at (null)
...
... skipping 50 lines of output
...
Creating block device ram8
Creating block device ram9
Creating Remain Block Devices
Making device-mapper control node
Scanning logical volumes
  Reading all physical volumes.  This may take a while...
  No volume groups found
  No volume groups found
Activating logical volumes
  No volume groups found
  No volume groups found
Free memory/Total memory (free %): 58272 / 116616 ( 49.9691 )
Saving to the local filesystem UUID=e7abcdeb-1987-4c69-a867-fabdceffghi2
Usage: fsck.ext4 [-panyrcdfvtDFV] [-b superblock] [-B blocksize]
        [-I inode_buffer_blocks] [-P process_inode_size]
        [-l|-L bad_blocks_file] [-C fd] [-j external_journal]
        [-E extended-options] device

Emergency help:
 -p                   Autom

  6. And then the system reboots (which is the default).

  7. When the system comes back online, there is nothing in /var/crash. I assume that the crash dump was not written.

    [root@host1 ~]# ls -lA /var/crash/
total 0
[root@host1 ~]#

  8. I know that crash dumps can work in general. If I tell kdump to copy the core dump to another system with the following configuration, kdump will successfully write the core dump to another host:

    path vmcore
ssh [email protected]
sshkey /root/.ssh/kdump_id_rsa

  9. If I set default shell in /etc/kdump.conf and rebuild initrd, and then crash the system again I get a slightly more informative error about mount: can't find /mnt in /etc/fstab

    Free memory/Total memory (free %): 58272 / 116616 ( 49.9691 )
Saving to the local filesystem UUID=e720481b-1987-4c69-a867-f2b4cba3b312
Usage: fsck.ext4 [-panyrcdfvtDFV] [-b superblock] [-B blocksize]
[-I inode_buffer_blocks] [-P process_inode_size]
[-l|-L bad_blocks_file] [-C fd] [-j external_journal]
[-E extended-options] device

Emergency help:
 -p                   Automatic repair (no questions)
 -n                   Make no changes to the filesystem
 -y                   Assume "yes" to all questions
 -c                   Check for bad blocks and add them to the badblock list
 -f                   Force checking even if filesystem is marked clean
 -v                   Be verbose
 -b superblock        Use alternative superblock
 -B blocksize         Force blocksize when looking for superblock
 -j external_journal  Set location of the external journal
 -l bad_blocks_file   Add to badblocks list
 -L bad_blocks_file   Set badblocks list
mount: can't find /mnt in /etc/fstab
dropping to initramfs shell
exiting this shell will reboot your system
/sys/block #

  10. But now, I'm stuck.

linux

2 Answers

  1. A little late to the game but if you need to configure kdump for the future:

    I think the path directive designates a path from the partition or file system designated. By default this is the root FS. If you have a separate partition in /etc/fstab for /var it will obfuscate the crash directory when your system is booted normally. ie if you were to boot normally and unmount /var you would see the crash/[UniqCoreDir]. You can adjust this by adding an ext4 /PATH/TO/DEVICE directive in kdump.conf. Also you could use a different path that won't be mounted over.

    Just a guess but might have a number of vmcores burried under /var.

    • 5

  2. Pull apart your kdump initrd in /boot/ check to to see the final path that its trying to dump to.

    • I think the "path" option is a little weird, I'd probably leave it to the default or set it explicitly to /var/crash

    • Do you have some kind of watchdog rebooting the machine ? this may also prevent the core being created by rebooting the machine before the is started.

    • 2