ubuntu-core-boot-modes

On an Ubuntu Core system, when a new kernel snap or core snap environment is installed, Ubuntu Core enables a special boot mode called try.

If the system fails to boot, try boot mode allows the system to revert automatically to a last known good core and kernel combination

Details

The implementation of try-boot touches both snapd and the bootloader (grub/uboot/aboot) in the following ways:

  1. By default, snap_mode is "", in which case the bootloader loads loads two squashfs files denoted by variables snap_core and snap_kernel.

  2. When a core/kernel snap is refreshed, snapd will set both snap_mode=try and snap_try_{core,kernel} to the core/kernel that will be tried next.

  3. On reboot, the bootloader will inspect snap_mode and, if it’s set to try, will set snap_mode=trying and attempt to boot the snap_try_{core,kernel}.

  4. On a successful boot, snapd resets snap_mode to "" and copies snap_try_{core,kernel} to snap_{core,kernel}. The snap_try_/* values are cleared afterwards. The bootmode is now in state(1) again.

  5. On a failing boot, the bootloader will see snap_mode=trying which means snapd did not start successfully. In this, case the bootloader will set snap_mode="" and the system will boot with the known good values from snap_{core,kernel}. The bootmode is in state(1) again.