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Chapter 18. Firmware assisted dump mechanisms

Firmware assisted dump (fadump) is a dump capturing mechanism, provided as an alternative to the kdump mechanism on IBM POWER systems. The kexec and kdump mechanisms are useful for capturing core dumps on AMD64 and Intel 64 systems. However, some hardware, such as mini systems and mainframe computers, uses the onboard firmware to isolate regions of memory and prevent any accidental overwriting of data that is important to the crash analysis. The fadump utility is optimized for the fadump mechanisms and their integration with RHEL on IBM POWER systems.

18.1. Firmware assisted dump on IBM PowerPC hardware
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The fadump utility captures the vmcore file from a fully-reset system with PCI and I/O devices. This mechanism uses firmware to preserve memory regions during a crash and then reuses the kdump userspace scripts to save the vmcore file. The memory regions consist of all system memory contents, except the boot memory, system registers, and hardware Page Table Entries (PTEs).

The fadump mechanism offers improved reliability over the traditional dump type, by rebooting the partition and using a new kernel to dump the data from the previous kernel crash. The fadump requires an IBM POWER6 processor-based or later version hardware platform.

For further details about the fadump mechanism, including PowerPC specific methods of resetting hardware, see the /usr/share/doc/kexec-tools/fadump-howto.txt file.

Note

The area of memory that is not preserved, known as boot memory, is the amount of RAM required to successfully boot the kernel after a crash event. By default, the boot memory size is 256MB or 5% of total system RAM, whichever is larger.

Unlike kexec-initiated event, the fadump mechanism uses the production kernel to recover a crash dump. When booting after a crash, PowerPC hardware makes the device node /proc/device-tree/rtas/ibm.kernel-dump available to the proc filesystem (procfs). The fadump-aware kdump scripts, check for the stored vmcore, and then complete the system reboot cleanly.

18.2. Enabling firmware assisted dump mechanism
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You can enhance the crash dumping capabilities of IBM POWER systems by enabling the firmware assisted dump (fadump) mechanism.

In the Secure Boot environment, the GRUB boot loader allocates a boot memory region, known as the Real Mode Area (RMA). The RMA has a size of 512 MB, divided among the boot components. If a component exceeds its size allocation, GRUB fails with an out-of-memory (OOM) error.

Warning

Do not enable firmware assisted dump (fadump) mechanism in the Secure Boot environment on RHEL 9.1 and earlier versions. The GRUB boot loader fails with the following error: 

error: ../../grub-core/kern/mm.c:376:out of memory.
Press any key to continue…
Copy to Clipboard Toggle word wrap

The system is recoverable only if you increase the default initramfs size due to the fadump configuration.

For information about workaround methods to recover the system, see the System boot ends in GRUB Out of Memory (OOM) article.

Prerequisites

  • You have root permissions on the system.

Procedure

  1. Install the kexec-tools package.

  2. Configure the default value for crashkernel. 

    # kdumpctl reset-crashkernel --fadump=on --kernel=ALL
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  3. Optional: Reserve boot memory instead of the default value. 

    # grubby --update-kernel ALL --args="fadump=on crashkernel=xxM"
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    xxM is the required memory size in megabytes.

    Note

    When specifying boot configuration options, test the configurations by rebooting the kernel with kdump enabled. If the kdump kernel fails to boot, increase the crashkernel value gradually to set an appropriate value.

  4. Reboot for changes to take effect. 

    # reboot
    Copy to Clipboard Toggle word wrap

18.3. Firmware assisted dump mechanisms on IBM Z hardware
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IBM Z systems support the following firmware assisted dump mechanisms:

  • Stand-alone dump (sadump)
  • VMDUMP

The kdump infrastructure is supported and utilized on IBM Z systems. However, using one of the firmware assisted dump (fadump) methods for IBM Z has the following benefits:

  • The system console initiates and controls the sadump mechanism, and stores it on an IPL bootable device.
  • The VMDUMP mechanism is similar to sadump. This tool is also initiated from the system console, but retrieves the resulting dump from hardware and copies it to the system for analysis.
  • These methods (similarly to other hardware based dump mechanisms) have the ability to capture the state of a machine in the early boot phase, before the kdump service starts.
  • Although VMDUMP contains a mechanism to receive the dump file into a Red Hat Enterprise Linux system, the configuration and control of VMDUMP is managed from the IBM Z Hardware console.

18.4. Using sadump on Fujitsu PRIMEQUEST systems
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The Fujitsu sadump mechanism provides a fallback dump capture when kdump is unable to complete successfully. You can manually invoke sadump from the system Management Board (MMB) interface. Using MMB, configure kdump like for an Intel 64 or AMD64 server and then proceed to enable sadump.

Procedure

  1. Add or edit the following lines in the /etc/sysctl.conf file to ensure that kdump starts as expected for sadump: 

    kernel.panic=0
kernel.unknown_nmi_panic=1
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    Warning

    In particular, ensure that after kdump, the system does not reboot. If the system reboots after kdump has failed to save the vmcore file, then it is not possible to invoke the sadump.

  2. Set the failure_action parameter in /etc/kdump.conf appropriately as halt or shell. 

    failure_action shell
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