Initrd for Ubuntu Core

This repository contains source code to build initial ramdisk used in Ubuntu Core Embedded Linux OS.

Purpose

See: https://en.wikipedia.org/wiki/Initial_ramdisk

Architecture

In Ubuntu Core, initrd.img file is part of Kernel Snap as a binary. This file is brought to Kernel snap from a PPA. See integration below.

In UC20 and further, initrd is migrated from script based
implementation (UC16/18) to systemd based. See the architecture
document for more details.

Building initrd for Ubuntu Core

TODO: Write documentation for how to build initrd locally

Prequisities

Preparation

Building

Testing & Debugging

See Hacking

Releasing

The UC initrd is included in kernel snaps. However, the process to get
it inside the kernel snap is not immediate and consists of a few
steps. First, we need to build the ubuntu-core-initramfs
debian package in the snappy-dev/image
PPA by
following these steps:

Update the changelog with latest changes since last release (use dch -i for this)
Commit using debcommit --release -a which will commit the changelog update & tag the release
Propose a PR to the repo with the new changelog, get it reviewed and merged
Push the tag to the repository with the new version (GitHub pull requests do not update tags)
Build the source package by running (note that the clean command
removes all untracked files, including subtrees with .git folders)
```
 git clean -ffdx
 gbp buildpackage -S -sa -d --git-ignore-branch
```

Compare with the latest package that was uploaded to the snappy-dev
PPA to make sure that the changes are correct. For this, you can
download the .dsc file and the tarball from the PPA, then run debdiff
to find out the differences:

 dget https://launchpad.net/~snappy-dev/+archive/ubuntu/image/+sourcefiles/ubuntu-core-initramfs/<old_version>/ubuntu-core-initramfs_<old_version>.dsc
 debdiff ubuntu-core-initramfs_<old_version>.dsc ubuntu-core-initramfs_<new_version>.dsc > diff.txt

Upload, or request sponsorship, to the snappy-dev PPA

 dput ppa:snappy-dev/image ubuntu-core-initramfs_<new_version>_source.changes

Make sure that the package has been built correctly. If not, make
changes appropriately and repeat these steps, including creating a
new changelog entry.

Note that ubuntu-core-initramfs gets some files from its build
dependencies while being built, including for instance
snap-bootstrap, so we need to make sure that the snappy-dev PPA
already contains the desired version of the snapd deb package (or
others) when we upload the package.

After this, the initrd changes will be included in future kernel snaps
releases automatically, following the usual 3 weeks cadence, as the
snappy-dev PPA is included when these build happen.

Updating systemd

git-subtree is used to synchronize with Ubuntu systemd. To update it, run

git subtree pull --prefix vendor/systemd https://git.launchpad.net/ubuntu/+source/systemd ubuntu/<series>-updates --squash

where series is the ubuntu series. Note that we are pulling from the
updates pocket, that is usually what we want, but it could be
different depending on the stage of development of the series.

To create the subtree from scratch, which is useful when bumping to a
new series, run

git rm -r vendor/systemd
git commit -a -m "Remove <old_series> systemd"
git subtree add --prefix vendor/systemd https://git.launchpad.net/ubuntu/+source/systemd ubuntu/<series> --squash

where we use the default pocket although again it might vary.

Bootchart

It is possible to enable bootcharts by adding ubuntu_core.bootchart
to the kernel command line. The sample collector will run until the
systemd switches root, and the chart will be saved in /run/log. If
bootcharts are also enabled for the core snap, that file will be
eventually moved to the ubuntu-save partition (see Core snap
documentation).

zyga/core-initrd