The switch from initrd to initramfs for newer Debian kernels

?TableOfContents()

Debian kernels require an initial ramdisk to work

Debian has for some time distributed and installed modular kernels by default, depending on the helper package initrd-tools to add the needed modules in a cramfs initrd-image.

2.6 has a newer format, initramfs, which is an cpio gzipped archive. The handoff to early userspace is much earlier. Initramfs allows a cleaner interface. The initrd hacks for nfs root and such are scheduled to be removed for 2.7.

Initrd-tools relies on devfs and on removed or deprecated /proc interfaces. It has no maintainer. Its ?CodingStyle is weird. There is no support for sata thus responsible for many sarge install failures nor scsi for kernels >= 2.6.10. Its use with recent 2.6 linux kernels is discouraged.

Several tools produce initramfs images, each with some spiffy features and annoying limitations...

Comparison of the various replacements for initrd-tools

Architectures

["initramfs-tools"]

["Yaird"]

amd64

(./) Works (MarcoAmadori, fs)

(./) Works (MarcoAmadori)

arm

{i} Needs Test

{i} Needs Test

armeb

(./) Works (tbm)

{i} Needs Test

alpha

(./) Works (nobse)

{i} Needs Test (Uwe Schindler)

hppa

(./) Works (kyle)

{i} Needs Test

i386

(./) Works (Jeff Bailey)

<!> Works, but no upgrade path from Sarge (JonasSmedegaard)

ia64

(./) Works (lamont)

<!> Needs Testing

m68k

{i} Needs Test

{i} Needs Test

mips

{X} klibc stat() needs work

{i} Needs Test

mipsel

{i} Needs Test

{i} Needs Test

powerpc

(./) Works (DannFrazier?)

(./) Works (?SvenLuther)

s390

(./) Works (?FransPop)

<!> Might work with manual editing config (see [http://bugs.debian.org/340344 #340344])

sh

{X} no klibc (unstested?)

{i} Needs Test

sparc

(./) Works (?JeffBailey)

(./) Works (JurijSmakov)

Root Devices

initramfs-tools

yaird

fstab label and UUID

(./) Works?FootNote(The root partition is passed as a parameter from the bootloader cmdline and can easily be set to /dev/disc/by-label/whatever or /dev/disc/by-uuid/whatever (i.e. no need to parse fstab))

<!> only ext2 and reiser (see [http://bugs.debian.org/337065 #337065] for workaround)?FootNote(Fstab labels and UUID on yaird works but yuckilly: Needs to be rewritten to blkid, there are good ideas in http://bootutils.sourceforge.net/) (ErikKonijnenburg)

md (fs directly)

(./) Works (maximilianattems)

(./) Works (JonasSmedegaard)

["LVM"]

(./) Works (maximilianattems)

(./) Works (MarcoAmadori)

LVM-over-md

(./) Works (?DanJacobowitz)

(./) Works (MarcoAmadori)

["EVMS"] (fs directly)

(./) Works (Sesse)

(./) Works in 0.12 (MarcoAmadori)

LVM-over-EVMS

(./) Works (Sesse)

(./) Works in 0.12 (MarcoAmadori)

md-over-EVMS

(./) Works (Sesse)

(./) Works in 0.12 (MarcoAmadori)

LVM-over-md-over-EVMS

(./) Works (Sesse)

(./) Works in 0.12 (MarcoAmadori)

Cryptsetup (plain)

<!> Needs cryptsetup with initramfs hooks (pending, see [http://bugs.debian.org/358452 #358452])

{i} Works (?ManojSrivasta) not for all (see [http://bugs.debian.org/336599 #336599])

Cryptsetup (luks)

<!> Needs cryptsetup with initramfs hooks (pending, see [http://bugs.debian.org/358452 #358452])

(./) Works (?MarcusBetter)

loopaes

{i} Needs Test

{X} No

["NFS"]

<!> Requires manual editing config file (?JeffBailey)

<!> Requires manual editing config file (JonasSmedegaard)

["SCSI"]

(./) Works (?JeffBailey)

{i} Needs test

["SATA"]

(./) Works (?JeffBailey)

(./) Works (MarcoAmadori)

USB-stick

(./) Works (maximilianattems)

{i} Needs Test

IEEE1394

(./) Works (mikap)

{X} No

Compaq Smart Arrays

<!> Grub's update-grub adds '!' in the devicename, once you remove those it works.

(./) added in 0.0.11-11, see [http://bugs.debian.org/335601 #335601]

dmraid

{i} Needs test, hooks can be found [https://wiki.ubuntu.com/FakeRaidHowto#head-2316ce782c999621d013c90ad3d4cc61349b46db here]

<!> In progress, patch see [http://bugs.debian.org/337724 #337724]

Other features

initramfs-tools

yaird

cramfs initrd

{X} No.

{X} Removed in 0.0.12, any reason to put it back? (ErikKonijnenburg)

USB keyboard

(./) Works (?TommiVainikainen)

(./) Works (ErikKonijnenburg)

swsusp

(./) Works (DavidHärdeman)

{X} No

swsusp2 ?FootNote(swsusp2 is not in mainline kernel)

{i} Needs test

<!> requires manual editing config files, and needs improvements (see [http://bugs.debian.org/329319 #329319])

External hooks

(./) Yes

<!> Requires manual editing config file

Package requirements (risk of delays on some archs)

initramfs-tools

yaird

klibc

<!> Yes

(./) No

busybox

<!> Yes

(./) No

["udev"]

<!> Yes

(./) No

perl

(./) No

<!> Yes

HTML::Template

(./) No

<!> Yes

Parse::?RecDescent

(./) No

<!> Yes

Arch: any

(./) No

<!> Yes (includes a few tiny C-based helper tools)

Runtime requirements

initramfs-tools

yaird

sysfs?FootNote(sysfs required at runtime means to depend on a mounted /sys tree. This limits to installing only '''from''' 2.6.8 and newer - specifically making upgrades directly from Linux 2.4.x to Linux 2.6.x impossible (Debian Sarge installed 2.4.x kernel by default on many arches!))

<!> Only sometimes?FootNote(sysfs is required for initramfs-tools at runtime in "dep" mode, but not in "most" mode (the default mode in Debian). Images created in "most" mode might be too big for some arches or bootloaders.)

<!> Yes

Boot-time requirements

initramfs-tools

yaird

udev?FootNote(udev required at boot-time limits to installing only Linux 2.6.12 and newer)

<!> Yes

(./) No

Legend

(./)

Great

<!>

Not great

{X}

Bad

{i}

Unknown, needs to be tested

Only some kernel versions work

Recent Linux kernels query the ramdisk tool prior to use if it is expected to succeed. Currently the following simplified queries try to handle all of the above contraints (see ?FlexibleKernelHandling for a future more finegrained approach):

kernel-package query

initramfs-tools

yaird

Supported host version

any

>=2.6.8

Supported target version

>=2.6.12

>=2.6.8

Comparable hilites beyond simply working

The main goal of all initrd tools is off course to generate an initial ramdisk to help boot a kernel properly. But beyond that, the tools have fundamental differences in their approach:

Hilite

initramfs-tools

yaird

howto compose images

Builds generic image with plenty of included modules, and probes at boot time what to load

Builds specific image probing/resolving with sysfs at build time the modules to include and load at boot time

Ramdisk size

Relatively large by default- option of relatively small (with larger risk of failing)

quite small

Dealing with MD (lvm, md, evms)

Activates all md, evms devices and all logical volumes of the root volume group.

Activates only md and/or LVM volume needed for mounting rootfs

Dealing with errors at image build time

Creates per default an generic image without probing.

Refuses to build image if uncertain it will work.

Dealing with errors at boot time

Drops into a shell to let you fix stuff.

No repair possibility on boot.

Copying root

Works out of the box. Noticed on boot by the new root bootparam.

Needs a newly regenerated initramfs.

libc used

Both klibc and (externally pulled in) glibc

glibc (can be compiled against klibc if needed)

Hilites not directly comparable

initramfs-tools

yaird