GRUB

COS is set to deploy a persistent grub.cfg into the COS_RECOVERY partition during the system installation or image creation. COS grub configuration includes three menu entries: first for the main OS system, second for the fallback OS system and a third for the recovery OS.

For example the main OS system menu entry could be something like:

menuentry "cOS" --id cos {
  search.fs_label COS_STATE root
  set img=/cOS/active.img
  set label=COS_ACTIVE
  loopback loop0 /$img
  set root=($root)
  source (loop0)/etc/cos/bootargs.cfg
  linux (loop0)$kernel $kernelcmd
  initrd (loop0)$initramfs
}

Specifying default custom boot options

cOS images and its derivatives, are expected to include a /etc/cos/bootargs.cfg file which provides the definition of the following variables:

• $kernel: Path of the kernel binary
• $kernelcmd: Kernel parameters
• $initramfs: Path of the initrd binary

This is the mechanism any cOS image or cOS derivative has to communicate its boot parameters (kernel, kernel params and initrd file) to GRUB2.

For example, the default cOS bootarg.cfg file is:

set kernel=/boot/vmlinuz
if [ -n "$recoverylabel" ]; then
    # Boot arguments when the image is used as recovery
    set kernelcmd="console=tty1 root=live:CDLABEL=$recoverylabel rd.live.dir=/ rd.live.squashimg=$img panic=5"
else
    # Boot arguments when the image is used as active/passive
    set kernelcmd="console=tty1 root=LABEL=$label iso-scan/filename=$img panic=5 security=selinux rd.cos.oemlabel=COS_OEM selinux=1"
fi

set initramfs=/boot/initrd

You can tweak that file to suit your needs if you need to specify persistent boot arguments.

Grub environment variables

cOS (since v0.5.8) makes use of the GRUB2 environment block which can used to define persistent GRUB2 variables across reboots.

Use grub2-editenv command line utility to define the desired values.

For instance use the following command to reboot to recovery system only once:

> grub2-editenv /oem/grubenv set next_entry=recovery

Default boot entry

The default grub configuration loads the /grubenv of any available device and evaluates on next_entry variable and saved_entry variable. By default none is set.

The default boot entry is set to the value of saved_entry, in case the variable is not set grub just defaults to the first menu entry.

next_entry variable can be used to overwrite the default boot entry for a single boot. If next_entry variable is set this is only being used once, GRUB2 will unset it after reading it for the first time. This is helpful to define the menu entry to reboot to without having to make any permanent config change.

Use grub2-editenv command line utility to define desired values.

For instance use the following command to reboot to recovery system only once:

> grub2-editenv /oem/grubenv set next_entry=recovery

Or to set the default entry to fallback system:

> grub2-editenv /oem/grubenv set saved_entry=fallback

Boot menu

By default cOS and derivatives shows the default boot menu entry while booting (cOS).

The grub menu entry is generated during installation and can be configured by setting GRUB_ENTRY_NAME in the cOS configuration file inside the derivative, or either via cloud-init before installation.

For example, specifying in /etc/cos/config:

GRUB_ENTRY_NAME=myOS

will automatically set the GRUB menu entries for active, passive and recovery to the specified value.

The grub menu boot entry can also be set with grub2-editenv:

> grub2-editenv /oem/grubenv set default_menu_entry=fooOS

Persistent boot option flags

It is possible to define persistent boot flag for each menu entry also via grub2-editenv:

• extra_active_cmdline: extra bootflags to be applied only on active boot
• extra_passive_cmdline: extra bootflags to be applied only on passive boot
• extra_recovery_cmdline: extra bootflags to be applied only on recovery
• extra_cmdline: will be applied to each boot entry

Customizing fallback logic

By default cOS boots into active, and if there are failures will boot into the passive, and finally if keeps failing, will boot into recovery.

It is possible to override the default fallback logic by setting default_fallback as grub environment, consider for example:

> grub2-editenv /oem/grubenv set default_fallback="2 0 1"

Will set the default fallback to “2 0 1” instead of the default “0 1 2”.