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# 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](#integrating-with-kernel-snap) below.
In UC20 and further, initrd is migrated from script based
implementation (UC16/18) to **systemd based**. See the [architecture
document](ARCHITECTURE.md) for more details.
# Building initrd for Ubuntu Core
TODO: Write documentation for how to build initrd locally
## Prequisities
## Preparation
## Building
# Testing & Debugging
See [Hacking](HACKING.md)
# 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](https://launchpad.net/~snappy-dev/+archive/ubuntu/image) by
following these steps:
1. Update the changelog with latest changes since last release (use `dch -i` for this)
1. Commit using `debcommit --release -a` which will commit the changelog update & tag the release
1. Propose a PR to the repo with the new changelog, get it reviewed and merged
1. Push the tag to the repository with the new version (GitHub pull requests do not update tags)
1. 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
1. 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
1. Upload, or request sponsorship, to the snappy-dev PPA
dput ppa:snappy-dev/image ubuntu-core-initramfs_<new_version>_source.changes
1. 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 saving samples in a file
under `/run/log/` until systemd switches root. This file will be moved then to
`/var/log/debug/boot<N>/`, `<N>` being the boot number since bootcharts were
enabled (saved together with the log run after switch root, see base snaps
documentation).
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