Ubuntu Core boot modes

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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.
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Would this be more relevant to the Ubuntu Core docs than the Snap docs?

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@mpt I think so - AIUI the medium/long term plan is to feed docs.ubuntu.com with content from the forum. Not sure what the best short term approach is though.

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FWIW as we add more Ubuntu Core docs, we’ll probably grow an ad-hoc section in the outline.

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This document has 2 titles - the topic title, “Boot modes in Ubuntu Core”, and an <h1> inside the topic itself of “Ubuntu Core boot modes”. This is different from how the other topics are written.

Could someone with edit rights please remove the <h1> from within the document?

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I think the best short-term approach is to remove the “doc” label so that this stops showing up on docs.snapcraft.io. Perhaps add an “ubuntucoredoc” or similar label instead.

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@degville I think this page should also have a special URL, currently it just shows up as https://snapcraft.io/docs/t/boot-modes-in-ubuntu-core/5464. I don’t think it needs to be in the side-bar though.

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Thanks for this - I’ve moved it into the special URL category. I also think this page is likely to be moved to the wider Ubuntu Core documentation when it’s published.

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