Appending kernel parameters to boot instances

Appending kernel parameters to boot instances

The Bare Metal service supports passing custom kernel parameters to boot instances to fit users’ requirements. The way to append the kernel parameters is depending on how to boot instances.

Network boot

Currently, the Bare Metal service supports assigning unified kernel parameters to PXE booted instances by:

  • Modifying the [pxe]/pxe_append_params configuration option, for example:

    pxe_append_params = quiet splash
  • Copying a template from shipped templates to another place, for example:

    Making the modifications and pointing to the custom template via the configuration options: [pxe]/pxe_config_template and [pxe]/uefi_pxe_config_template.

Local boot

For local boot instances, users can make use of configuration drive (see Enabling the configuration drive (configdrive)) to pass a custom script to append kernel parameters when creating an instance. This is more flexible and can vary per instance. Here is an example for grub2 with ubuntu, users can customize it to fit their use case:

#!/usr/bin/env python
import os

# Default grub2 config file in Ubuntu
grub_file = '/etc/default/grub'
# Add parameters here to pass to instance.
kernel_parameters = ['quiet', 'splash']
old_grub_file = grub_file+'~'
os.rename(grub_file, old_grub_file)
cmdline_existed = False
with open(grub_file, 'w') as writer, \
       open(old_grub_file, 'r') as reader:
       for line in reader:
           key = line.split('=')[0]
           if key == grub_cmd:
               #If there is already some value:
               if line.strip()[-1] == '"':
                   line = line.strip()[:-1] + ' ' + ' '.join(kernel_parameters) + '"'
               cmdline_existed = True
       if not cmdline_existed:
           line = grub_cmd + '=' + '"' + ' '.join(kernel_parameters) + '"'



In order to change default console configuration in the Bare Metal service configuration file ([pxe] section in /etc/ironic/ironic.conf), include the serial port terminal and serial speed. Serial speed must be the same as the serial configuration in the BIOS settings, so that the operating system boot process can be seen in the serial console or web console. Following examples represent possible parameters for serial and web console respectively.

  • Node serial console. The console parameter console=ttyS0,115200n8 uses ttyS0 for console output at 115200bps, 8bit, non-parity, e.g.:

    # Additional append parameters for baremetal PXE boot.
    pxe_append_params = nofb nomodeset vga=normal console=ttyS0,115200n8
  • For node web console configuration is similar with the addition of ttyX parameter, see example:

    # Additional append parameters for baremetal PXE boot.
    pxe_append_params = nofb nomodeset vga=normal console=tty0 console=ttyS0,115200n8

For detailed information on how to add consoles see the reference documents kernel params and serial console. In case of local boot the Bare Metal service is not able to control kernel boot parameters. To configure console locally, follow ‘Local boot’ section above.

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