mkinitcpio/Minimal initramfs
This article shows how to create a slim, minimal initramfs for a system with a specific, known and static hardware configuration. The procedure is expounded from Optimizing Bootup With mkinitcpio by Falconindy (Dave Reisner).
Udev requirement
The big advantage of creating your own initramfs images is that you can eliminate udev
. This hook alone is responsible for quite a bit of size (~700-800 KiB with LZ4 and LZOP, less with other algorithms) in the initramfs image. Not only will the bigger size lead to longer boots (more data to decompress) but initializing udev
itself will also take some extra time. However, some things require udev
. This includes resolving UUID, LABEL, PARTUUID and PARTLABEL identifiers (workaround hook without-udev) and the assembly of LVM and mdadm devices that contain the root
partition. If you are unsure if you need udev
, continue with the directions on this page up until the #Initial test. If not everything works without udev
, re-enable the hook and try again.
Also, while most keyboards (AT, PS/2, USB) do not require the use of the udev
hook, Logitech USB devices using the Logitech Unified Receiver do. At this point you could either include udev
in all images or rely on a fallback
image that does.
If you need udev
, your minimization efforts will most likely be in vain. You may still be able to shrink the image size by ~600 KiB, but boot times will not be significantly improved. Continuing on in this scenario can still be a worthwhile learning experience.
Editing .preset files
In Falconidy's tutorial, he edits /etc/mkinitcpio.conf
and runs mkinitcpio -g
to create the test initramfs image, leaving the known-good initramfs images on the system untouched. However, if you blindly run mkinitcpio -P
afterwards, even the fallback
image will be stripped down.
A safer way to prepare for taking the creation of the initramfs files into your own hands is to modify the .preset
files in /etc/mkinitcpio.d
. The following example configuration will supplant default
with the minimal initramfs image and create a new normal
image that is built The Arch Way. If things go wrong, you can rely on the normal
or fallback
images. When you are finished, you can drop the normal_*
lines from the configuration and remove the initramfs-linux*-normal.img
files.
... PRESETS=('default' 'normal' 'fallback') ... default_options="-S udev,block,mdadm_udev,filesystems,keyboard,fsck,consolefont" ... #normal_config="/etc/mkinitcpio.conf" normal_image="/boot/initramfs-linux-normal.img" #normal_options="" ...
mdadm_udev
and consolefont
hooks are not present in the default Arch configuration. Having extraneous hooks in the -S
parameter in the *_options
line will not result in an error.Finding needed modules
The quickest way to find out what modules you need is to reboot your system with the fallback
initramfs image and add break=postmount
to the kernel parameters in your boot loader so you get dropped to the command line once the root filesystem is mounted.
Once your system reboots, run the following command to see what modules you need:
lsmod | awk 'NF==3{print $1}'
awk
command returns only the first field, using {print $1}
, of every line with exactly 3 fields, enforced by NF==3
. Module dependencies include the 4th field to show which module pulled in the dependency, thus being filtered out due to that fourth field. Arch's mkinitcpio
takes care of dependencies for legitimate values included in the arrays MODULES=()
, FILES=()
, and BINARIES=()
.Write down the modules that were loaded and type exit
to continue booting.
Alternatively, Install the package hwdetect to help determine necessary modules. Though unmaintained, it can provide valuable information. Also, see Kernel modules to get started with the native tools.
Initial edit of mkinitcpio.conf
Edit /etc/mkinitcpio.conf
and modify the MODULES=
array. A worthwhile note is that /etc/mkinitcpio.conf
is sourced, so you can build the MODULES array like in a bash script.
MODULES=() # filesystems MODULES+=() # storage MODULES+=() # keyboard MODULES+=() # miscellaneous
Add all your modules to the last miscellaneous
line. As you sort through your modules, you can place them in the appropriate line.
You will also need the binaries to do filesystem checks on the root
device and any other mount points in /etc/fstab
that have been set to do so.
- For ext[2|3|4] devices:
BINARIES=(fsck fsck.ext[2|3|4] e2fsck)
- For vfat (UEFI boot) partitions:
BINARIES=(fsck fsck.vfat dosfsck)
- For btrfs single disk device:
BINARIES=(fsck fsck.btrfs btrfsck)
- For btrfs multi disk device:
BINARIES=(fsck fsck.btrfs btrfs btrfsck)
- For xfs devices
BINARIES=(fsck fsck.xfs xfs_repair)
- The third option in each of these example are optional, but their exclusion will prevent you from repairing a damaged filesystem, necessitating a boot from another initramfs.
- Users are encouraged to add entries pertaining to other filesystems.
Initial test
Edit /etc/mkinitcpio.conf
and run mkinitcpio -P
to rebuild all of your initramfs images. Then reboot.
Your first boot should be successful if you do not need udev
. If something does not work (eg, Arch cannot find your root partition or your keyboard does not work) then you will need to go back and remove udev
from the -S
parameter in the default_options
line and try again. If you need udev
, keep in mind that you will not see a significant improvement in boot time and continuing on is only good for a learning experience.
Sorting out modules
Now that you have a known-good bootable initramfs, it is time to slim down the initramfs even further. The normal method is to remove a few modules at a time, rebuild the initramfs images, and reboot to see if everything is still OK. If you find out that everything is not OK, reboot with the fallback
initramfs image and re-add the deleted modules until everything is OK again. Rinse and repeat until you have only the modules you need. As this can be a tedious experience, the following lists are provided to give people a head-start in the elimination process.
Filesystem modules
root
device and any other device in /etc/fstab
that will have its filesystem checked on boot.ext[2,3,4]
xfs
jfs
reiserfs
Storage device modules
sd_mod
for all SCSI, SATA, and PATA (IDE) devicesahci
for SATA devices on modern AHCI controllersnvme
andnvme_core
for NVMe (M.2, PCI-E) devicessata_*
for SATA devices on IDE-mode controllerspata_*
for PATA (IDE) devicesehci_pci
andusb_storage
for USB storage devicesvirtio_blk
andvirtio_pci
for QEMU/KVM VMs using VirtIO for storage
Keyboard modules
atkbd
for AT and PS/2 keyboards, and the emulated keyboard in QEMU/KVM.hid_generic
,ohci_pci
, andusbhid
for normal USB keyboards.hid_logitech_dj
,uhci_hcd
, andusbhid
for Logitech USB keyboards using the Logitech Unified Receiver (requires theudev
hook).
Finishing up
Once you have slimmed your initramfs as far as it will go, remove (or comment-out) the normal_*
lines from your .preset
files and remove the initramfs-linux*-normal.img
files from /boot
.