Abstract
Switch the default initrd infrastructure from initramfs-tools to Dracut.
Rationale
Ubuntu desktop/server uses initramfs-tools to generate initrds which has following problems/drawbacks:
- systemd is not used in the initrd (for point 3 and 5)¹
- It lacks features like NVMe over Fabric (NVM-oF) support
- There is not much code sharing between the initrd and the root system, increasing maintenance costs
- There is not much development work done upstream in the recent years (mostly maintenance work)
- There is a lot of Shell code in the initrd²
- The initrd generation is pure Shell code²
¹ Ubuntu Core is already using a systemd-based initrd (core-initrd uses Dracut).
² Shell code makes it slow, fragile, hard to understand, and all the usual issues with shell scripts.
Specification
To address the first four points of the Rationale the plan is to change the default initrd generation tool of Ubuntu Desktop and Ubuntu server from initramfs-tools to Dracut. Since systemd is used in Ubuntu, Dracut will use the systemd Dracut module. Using systemd in the initrd will reduce the amount of Shell code in the initrd.
Dracut will provide support for following features (compared to initramfs-tools):
- NVMe over Fabric (NVMF) support
- Bluetooth keyboard support during LUKS disk unlocking
This switch will be done step by step over multiple Ubuntu releases:
- Make sure that the common/basic use cases work with Dracut. Allow users testing Dracut for their setup (by installing dracut-core and calling
dracutmanually). Provide instructions for testing and reporting issues found there. Collect feedback which use cases need work. This is planned for Ubuntu 24.10 (oracular). - Support Dracut as alternative initrd tool: Promote dracut to main (MIR bug #2031304) and change initramfs-tools dependencies in ubuntu-desktop (and similar) to
initramfs-tools | dracut(see Support Dracut as alternative initrd tool). Maintainers of prebuilt images can decide to use dracut (e.g. some cloud images). This is planned for Ubuntu 25.04. - Make Dracut the default initrd tool by changing dependencies in ubuntu-desktop (and similar) to `
dracut | initramfs-tools`. The installer will install dracut by default. Users can still install initramfs-tools in case of problems. This is proposed for Ubuntu 25.10. - Drop support for initramfs-tools (demote to universe) and only support Dracut. ubuntu-desktop (and similar) will only depend on `dracut`. This is proposed for Ubuntu 26.04.
Issues and missing support for use cases will be tracked on https://bugs.launchpad.net/ubuntu/+source/dracut. Each step can be deferred to a later Ubuntu release in case there are issues reported that need to be addressed first.
The live images use casper and initramfs-tools. These live images need porting to Dracut which supports live systems via dracut-live. Dracut-live supports booting from SquashFS images by specifying root=live:<URL> (where <URL> could be CDLABEL=$ubuntu_label). Dracut needs to be extended to support stacking multiple squashfs images (see upstream bug #700). All casper-bottom scripts need to be ported to Dracut or incorporated into the respective upstream projects (e.g. services could be enabled/disabled via ConditionKernelCommandLine=rd.live.image or similar).
Reducing the amount of Shell code can be done afterwards in the Dracut upstream project, which already has some C code (dracut-install and dracut-utils). Efforts to port more Shell code to a proper language are more likely in Dracut due to the wider community.
Moving to a vendor-built initrd is orthogonal to this spec and is not part of it.
Supported and tested use cases
All disk setup options that you can select in the installer need to be supported by Dracut before making Dracut the default (see point 3):
- default (ext4)
- LVM
- LVM with encryption
- ZFS
- ZFS with encryption: dracut does not support booting from an encrypted ZFS volume (LP: #2070066)
We will officially only support Dracut when used together with the systemd module. Dracut modules for packages in universe will only get community support.
packages with initramfs-tools integration
Ubuntu ships 77 packages that ship initramfs-tools hooks/scripts/modules:
| Source | Package | Component | dracut-support |
|---|---|---|---|
| ntfs-3g | ntfs-3g | main | maybe, ntfs-3g is supported in 90dmsquash-live |
| brltty | brltty | main | no, brltty and braille not found in source |
| busybox | busybox-initramfs | main | no, but 05busybox uses /usr/bin/busybox |
| casper | casper | main | no, needs porting, dracut supports live images |
| cloud-initramfs-tools | cloud-initramfs-copymods | main | no, copymods or module copying /lib/modules not found in source |
| cloud-initramfs-tools | cloud-initramfs-dyn-netconf | main | no, but should be covered by network module |
| cloud-initramfs-tools | cloud-initramfs-growroot | main | no, but cloud-initramfs-tools source has 50growroot module |
| cloud-initramfs-tools | cloud-initramfs-rescuevol | main | no, rescue not found (except rescue.service from systemd) |
| cloud-initramfs-tools | cloud-initramfs-updateroot | main | no, updateroot not found in source |
| kdump-tools | kdump-tools | main | no, needs porting |
| klibc | klibc-utils | main | no, klibc not found in source |
| mini-iso-tools | mini-iso-tools | main | no, needs porting |
| cloud-initramfs-tools | overlayroot | main | no, needs porting to 90overlayfs dracut module |
| ubuntu-raspi-settings | ubuntu-raspi-settings-desktop | main | no, needs porting, zswap not found in dracut source |
| walinuxagent | walinuxagent | main | no, but just copies 2 udev rules, azure not found in source |
| cloud-initramfs-tools | cloud-initramfs-rooturl | main | partial, root=live:<url> supports squashfs but not tarballs |
| fuse3 | fuse3 | main | partial, in 90dmsquash-live-ntfs module |
| amd64-microcode | amd64-microcode | main | yes, see early_microcode in dracut.sh |
| bcache-tools | bcache-tools | main | yes, package ships 90bcache dracut module |
| btrfs-progs | btrfs-progs | main | yes, in 90btrfs dracut module |
| cryptsetup | cryptsetup-initramfs | main | yes, 90crypt and 90systemd-cryptsetup and others |
| dmraid | dmraid | main | yes, 90dmraid module |
| lvm2 | dmsetup | main | yes, probably by 90dm module |
| intel-microcode | intel-microcode | main | yes, see early_microcode in dracut.sh |
| open-iscsi | iscsiuio | main | yes, in 95iscsi module |
| kbd | kbd | main | yes, in 10i18n module |
| console-setup | keyboard-configuration | main | yes, in 10i18n module but might need added support |
| kmod | kmod | main | yes, initramfs-tools hook not needed in dracut |
| multipath-tools | kpartx-boot | main | yes, in 90multipath module |
| lvm2 | lvm2 | main | yes, probably by 90lvm module |
| mdadm | mdadm | main | yes, in 90mdraid module |
| multipath-tools | multipath-tools-boot | main | yes, in 90multipath module |
| open-iscsi | open-iscsi | main | yes, in 95iscsi module |
| plymouth | plymouth | main | yes, in 50plymouth dracut module |
| reiserfsprogs | reiserfsprogs | main | yes, dracut supports reiserfsck |
| sg3-utils | sg3-utils-udev | main | yes, 95udev-rules copies the scsi-sg3 udev rules |
| thin-provisioning-tools | thin-provisioning-tools | main | yes, 90lvm supports thin volumes |
| systemd | udev | main | yes, in 01systemd-udevd dracut module |
| xfsprogs | xfsprogs | main | yes, dracut supports xfs |
| zfs-linux | zfs-initramfs | main | yes, via zfs-dracut |
| r8125 | r8125-dkms | multiverse | maybe, this hook just copies the r8125 kernel module |
| nvidia-graphics-drivers-470 | nvidia-kernel-common-470 | restricted | maybe, this hook just copies the nvidia kernel module |
| nvidia-graphics-drivers-470-server | nvidia-kernel-common-470-server | restricted | maybe, this hook just copies the nvidia kernel module |
| nvidia-graphics-drivers-535 | nvidia-kernel-common-535 | restricted | maybe, this hook just copies the nvidia kernel module |
| nvidia-graphics-drivers-535-server | nvidia-kernel-common-535-server | restricted | maybe, this hook just copies the nvidia kernel module |
| nvidia-graphics-drivers-550 | nvidia-kernel-common-550 | restricted | maybe, this hook just copies the nvidia kernel module |
| nvidia-graphics-drivers-550-server | nvidia-kernel-common-550-server | restricted | maybe, this hook just copies the nvidia kernel module |
| librem-ec-acpi | librem-ec-acpi-dkms | universe | maybe, this hook just copies the librem_ec_acpi kernel module |
| r8168 | r8168-dkms | universe | maybe, this hook just copies the r8168 kernel module |
| aoetools | aoetools | universe | no, aoe not found in source |
| bcachefs-tools | bcachefs-tools | universe | no, bcachefs not found in source |
| bilibop | bilibop-lockfs | universe | no, bilibop and lockfs not found in source |
| bootcd | bootcd | universe | no, but dracut supports live images |
| cryptsetup-nuke-password | cryptsetup-nuke-password | universe | no, nuke not found in source |
| dell-recovery | dell-recovery-casper | universe | no, it hooks into casper, dell not found in source |
| dropbear | dropbear-initramfs | universe | no, dropbear not found in source |
| initramfs-tools-devices | initramfs-tools-devices | universe | no, resize not found in source |
| jasper-initramfs | jasper | universe | no, no resize found in source |
| kubuntu-settings | kubuntu-settings-desktop | universe | no, needs porting (but is a simple one-liner) |
| kxd | kxc | universe | no, kxc not found in source |
| live-boot | live-boot-initramfs-tools | universe | no, needs porting to root=live:<url> |
| ltsp | ltsp | universe | no, ltsp not found in source |
| mandos | mandos-client | universe | no, mandos not found in source |
| open-infrastructure-system-tools | open-infrastructure-system-boot | universe | no, project has dracut backend but it contains FIXME |
| osk-sdl | osk-sdl | universe | no, osk not found in source |
| qcom-phone-utils | qcom-phone-utils | universe | no, neither mobile nor qcomsoc found in source |
| toybox | toybox | universe | no, toybox not found in source |
| ubuntukylin-theme | ubuntukylin-theme | universe | no, neither truetype nor droid found in source |
| ubuntustudio-default-settings | ubuntustudio-default-settings | universe | no, org.kde.discover.notifier.desktop can be disabled differently |
| unl0kr | unl0kr | universe | no, unl0kr not found in source |
| v86d | v86d | universe | no, v86d not found in source |
| xubuntu-default-settings | xubuntu-live-settings | universe | no, ubiquity not found in source |
| yubikey-luks | yubikey-luks | universe | no, yubikey not found in source |
| fuse | fuse | universe | partial, in 90dmsquash-live-ntfs module |
| acpi-override | acpi-override-initramfs | universe | yes, set acpi_override and acpi_table_dir |
| clevis | clevis-initramfs | universe | yes, via clevis-dracut |
| nbd | nbd-client | universe | yes, in 95nbd module |
Further Information
As of 2024-09-20 subiquity contains a proof of concept for installing Ubuntu with NVMe/TCP firmware support.
Considered options
Following solutions were considered:
- Switch to dracut
- Add support for systemd in initramfs-tools
- Switch to mkosi-initrd
Some of the reasons for switching to Dracut were mentioned above. Dracut is an upstream project which is used in several distributions. It has an active community and responsive maintainers. As of 2024-09-18 Benjamin Drung contributed 69 commits upstream and considers the upstream collaboration excellent.
Add support for systemd in initramfs-tools was decided against, because the expected amount of time for implementation is bigger than switching to Dracut. Also the maintenance work will be bigger, because initramfs-tools is a Debian-specific tool and the load cannot be shared with other distributions (except maybe Debian).
mkosi-initrd takes another approach than initramfs-tools/Dracut. It uses the distro packages to construct the initrd. Current status of mkosi-initrd: It works for simple use cases, but the initrd is still bigger than the alternatives. Upstream is focusing on the infrastructure first. It will be a long road to support all the different use cases. mkosi-initrd relies on the distro packages to be available and does not support running on installed distro packages. See also
the All Systems Go! 2024 talk “mkosi-initrd: initrds built from system packages”
So switching to mkosi-initrd is not viable in the short or medium term, but could be considered in the future. A smoother approach could be taken: Work on the upstream projects to work out of the box in the initrd. This will reduce the needed Shell script in Dracut but also pave the road to make these projects work with mkosi-initrd.
