- The initial ramdisk is in essence a very small environment (early userspace) which loads various kernel modules and sets up necessary things before handing over control to
init. This makes it possible to have, for example, encrypted root file systems and root file systems on a software RAID array. mkinitcpio allows for easy extension with custom hooks, has autodetection at runtime, and many other features.
Traditionally, the kernel was responsible for all hardware detection and initialization tasks early in the boot process before mounting the root file system and passing control to
init. However, as technology advances, these tasks have become increasingly complex.
Nowadays, the root file system may be on a wide range of hardware, from SCSI to SATA to USB drives, controlled by a variety of drive controllers from different manufacturers. Additionally, the root file system may be encrypted or compressed; within a software RAID array or a logical volume group. The simple way to handle that complexity is to pass management into userspace: an initial ramdisk. See also: /dev/brain0 » Blog Archive » Early Userspace in Arch Linux.
mkinitcpio has been developed by the Arch Linux developers and from community contributions. See the public Git repository.
Install the package, which is a dependency of the package, so most users will already have it installed.
Advanced users may wish to install the latest development version of mkinitcpio from Git with the package.
Image creation and activation
Every time a kernel is installed or upgraded, a pacman hook automatically generates a .preset file saved in . For example
linux.preset for the official stable kernel package. A preset is simply a list of information required to create initial ramdisk images, instead of manually specifying the various parameters and the location of the output files.
By default, it contains the instructions to create two images:
- the default ramdisk image created following the directives specified in the mkinitcpio #Configuration, and
- the fallback ramdisk image, same as above except that the autodetect hook is skipped during creation, thus including a full range of modules which supports most systems.
After creating the preset, the pacman hook calls the mkinitcpio script which generates the two images, using the information provided in the preset.
To run the script manually, refer to the mkinitcpio(8) manual page for instructions. In particular, to (re-)generate the preset provided by a kernel package, use the / option followed by the preset to utilize. For example, for the package, use the command:
# mkinitcpio -p linux
To (re-)generate all existing presets, use the / switch. This is typically used to regenerate all the initramfs images after a change of the global #Configuration:
# mkinitcpio -P
Users may create any number of initramfs images with a variety of different configurations. The desired image must be specified in the respective boot loader configuration file.
Users can generate an image using an alternative configuration file. For example, the following will generate an initial ramdisk image according to the directions in and save it as .
# mkinitcpio --config /etc/mkinitcpio-custom.conf --generate /boot/initramfs-custom.img
If generating an image for a kernel other than the one currently running, add the kernel release version to the command line. The installed kernel releases can be found in , the syntax is consistent with the output of the command for each kernel.
# mkinitcpio --generate /boot/initramfs-custom2.img --kernel 5.7.12-arch1-1
Unified kernel images
As of version 31, mkinitpcio can also create unified kernel images.
The primary configuration file for mkinitcpio is
/etc/mkinitcpio.conf. Additionally, preset definitions are provided by kernel packages in the
/etc/mkinitcpio.d directory (e.g. ).
Users can modify six variables within the configuration file, see for more details:
- Kernel modules to be loaded before any boot hooks are run.
- Additional binaries to be included in the initramfs image.
- Additional files to be included in the initramfs image.
- Hooks are scripts that execute in the initial ramdisk.
- Used to compress the initramfs image.
- Extra arguments to pass to the program. Usage of this setting is strongly discouraged. mkinitcpio will handle special requirements for compressors (e.g. passing
--check=crc32to xz), and misusage can easily lead to an unbootable system.
- Some hooks that may be required for your system like lvm2, mdadm_udev, and encrypt are NOT enabled by default. Read the #HOOKS section carefully for instructions.
- Users with multiple hardware disk controllers that use the same node names but different kernel modules (e.g. two SCSI/SATA or two IDE controllers) should use persistent block device naming to ensure that the right devices are mounted. Otherwise, the root device location may change between boots, resulting in kernel panics.
The array is used to specify modules to load before anything else is done.
Modules suffixed with a will not throw errors if they are not found. This might be useful for custom kernels that compile in modules which are listed explicitly in a hook or configuration file.
BINARIES and FILES
These options allow users to add files to the image. Both and are added before hooks are run, and may be used to override files used or provided by a hook. are auto-located within a standard and are dependency-parsed, meaning any required libraries will also be added. are added as-is. For example:
Note that as both and are Bash arrays, multiple entries can be added delimited with spaces.
The array is the most important setting in the file. Hooks are small scripts which describe what will be added to the image. For some hooks, they will also contain a runtime component which provides additional behavior, such as starting a daemon, or assembling a stacked block device. Hooks are referred to by their name, and executed in the order they exist in the array of the configuration file.
The default setting should be sufficient for most simple, single disk setups. For root devices which are stacked or multi-block devices such as LVM, RAID, or dm-crypt, see the respective wiki pages for further necessary configuration.
Build hooks are found in , custom build hooks can be placed in . These files are sourced by the bash shell during runtime of mkinitcpio and should contain two functions: and
help. The function describes the modules, files, and binaries which will be added to the image. An API, documented by mkinitcpio(8), serves to facilitate the addition of these items. The
help function outputs a description of what the hook accomplishes.
For a list of all available hooks:
$ mkinitcpio -L
Use mkinitcpio's / option to output help for a specific hook, for example:
$ mkinitcpio -H udev
Runtime hooks are found in
/usr/lib/initcpio/hooks/, custom runtime hooks can be placed in . For any runtime hook, there should always be a build hook of the same name, which calls to add the runtime hook to the image. These files are sourced by the busybox ash shell during early userspace. With the exception of cleanup hooks, they will always be run in the order listed in the setting. Runtime hooks may contain several functions:
: Functions of this name will be run once the API file systems have been mounted and the kernel command line has been parsed. This is generally where additional daemons, such as udev, which are needed for the early boot process are started from.
: Functions of this name are run shortly after the early hooks. This is the most common hook point, and operations such as assembly of stacked block devices should take place here.
: Functions of this name are run after the root device has been mounted. This should be used, sparingly, for further setup of the root device, or for mounting other file systems, such as .
: Functions of this name are run as late as possible, and in the reverse order of how they are listed in the array in the configuration file. These hooks should be used for any last minute cleanup, such as shutting down any daemons started by an early hook.
A table of common hooks and how they affect image creation and runtime follows. Note that this table is not complete, as packages can provide custom hooks.
|Runtime hook (busybox init only)
|Sets up all initial directories and installs base utilities and libraries. Always keep this hook as the first hook unless you know what you are doing, as it provides critical busybox init when not using systemd hook.
Optional when using the systemd hook as it only provides a busybox recovery shell.
|Adds udevd, udevadm, and a small subset of udev rules to your image.
|Starts the udev daemon and processes uevents from the kernel; creating device nodes. As it simplifies the boot process by not requiring the user to explicitly specify necessary modules, using it is recommended.
|Adds support for on a separate partition. See #/usr as a separate partition for details.
|Mounts the partition after the real root has been mounted.
|Tries to resume from the "suspend to disk" state. See Hibernation for further configuration.
|Sets the required modules to enable Btrfs for using multiple devices with Btrfs. You need to have installed to use this. This hook is not required for using Btrfs on a single device.
btrfs device scan to assemble a multi-device Btrfs root file system when udev hook or systemd hook is not present. The package is required for this hook.
|Shrinks your initramfs to a smaller size by creating a whitelist of modules from a scan of sysfs. Be sure to verify included modules are correct and none are missing. This hook must be run before other subsystem hooks in order to take advantage of auto-detection. Any hooks placed before 'autodetect' will be installed in full.
|Includes modprobe configuration files from
/etc/modprobe.d/ and .
|Adds GPU modules to provide early KMS start. Additionally adds modules that are required by privacy screens built into the LCD panel of some laptops.
|Adds all block device modules, formerly separately provided by the fw, mmc, pata, sata, scsi, usb, and virtio hooks.
|Adds the necessary modules for a network device. You must have installed to use this, see #Using net for details.
|Provides handling for an NFS-based root file system.
|Provides support for fakeRAID root devices. You must have installed to use this. Note that it is preferred to use mdadm with the mdadm_udev hook with fakeRAID if your controller supports it. See #Using RAID for details.
|Locates and assembles fakeRAID block devices using .
|Provides support for assembling RAID arrays via udev. You must have installed to use this. If you use this hook with a FakeRAID array, it is recommended to include
mdmon in . See #Using RAID for details.
|Adds the necessary modules for keyboard devices. Use this if you have an USB or serial keyboard and need it in early userspace (either for entering encryption passphrases or for use in an interactive shell). As a side effect, modules for some non-keyboard input devices might be added too, but this should not be relied on. Supersedes old usbinput hook.
|Adds the specified keymap(s) from
/etc/vconsole.conf to the initramfs. If you use system encryption, especially full-disk encryption, make sure you add it before the hook.
|Loads the specified keymap(s) from
/etc/vconsole.conf during early userspace.
|Adds the specified console font from
/etc/vconsole.conf to the initramfs.
|Loads the specified console font from
/etc/vconsole.conf during early userspace.
|Adds the kernel module and the
cryptsetup tool to the image. You must have installed to use this.
|Detects and unlocks an encrypted root partition. See #Runtime customization for further configuration.
For sd-encrypt see dm-crypt/System configuration#Using systemd-cryptsetup-generator.
|Adds the device mapper kernel module and the
lvm tool to the image. You must have installed to use this. This is necessary if you have your root file system on LVM.
|This includes necessary file system modules into your image. This hook is required unless you specify your file system modules in .
|Adds the fsck binary and file system-specific helpers to allow running fsck against your root device (and if separate) prior to mounting. If added after the autodetect hook, only the helper specific to your root file system will be added. Usage of this hook is strongly recommended, and it is required with a separate partition. It is highly recommended that if you include this hook that you also include any necessary modules to ensure your keyboard will work in early userspace.
The use of this hook requires the parameter to be set on the kernel command line (discussion). See fsck#Boot time checking for more details.
The kernel supports several formats for compression of the initramfs: , , lzma, xz, , and . mkinitcpio uses zstd compressed images by default, note that the zstd compression runs in multi-threading mode (with the
-T0 option which spawns as many threads as detected cores).
The provided has the various options commented out. Uncomment one if you wish to switch to another compression method and make sure you have the corresponding compression utility installed. If none is specified, the zstd default method is used. If you wish to create an uncompressed image, specify in the configuration file or use on the command line.
These are additional flags passed to the program specified by , such as:
This option can be left empty; mkinitcpio will ensure that any supported compression method has the necessary flags to produce a working image.
With the default zstd compression, to save space for custom kernels (especially with a dual boot setup when using the EFI system partition as ), the
--long option is very effective. However, systems with limited RAM may not be able to decompress initramfs using this option. The option may also be desired to see details during the initramfs generation. For example:
COMPRESSION_OPTIONS=(-v -5 --long)
Runtime configuration options can be passed to
init and certain hooks via the kernel command line. Kernel command-line parameters are often supplied by the bootloader. The options discussed below can be appended to the kernel command line to alter default behavior. See Kernel parameters and Arch boot process for more information.
init from base hook
- This is the most important parameter specified on the kernel command line, as it determines what device will be mounted as your proper root device. mkinitcpio is flexible enough to allow a wide variety of formats, for example:
root=/dev/sda1 # /dev node root=LABEL=CorsairF80 # label root=UUID=ea1c4959-406c-45d0-a144-912f4e86b207 # UUID root=PARTUUID=14420948-2cea-4de7-b042-40f67c618660 # GPT partition UUID
- If or is specified,
initpauses the boot process (after loading hooks, but before mounting the root file system) and launches an interactive shell which can be used for troubleshooting purposes. This shell can be launched after the root has been mounted by specifying . Normal boot continues after exiting from the shell.
- Disable hooks at runtime by adding . For example:
- Alter the order in which modules are loaded by specifying modules to load early via . (This may be used, for example, to ensure the correct ordering of multiple network interfaces.)
requires the package.
Comprehensive and up-to-date information can be found in the official kernel documentation.
This parameter tells the kernel how to configure IP addresses of devices and also how to set up the IP routing table. It can take up to nine arguments separated by colons:
If this parameter is missing from the kernel command line, all fields are assumed to be empty, and the defaults mentioned in the kernel documentation apply. In general this means that the kernel tries to configure everything using autoconfiguration.
The parameter can appear alone as the value to the parameter (without all the characters before). If the value is or , no autoconfiguration will take place, otherwise autoconfiguration will take place. The most common way to use this is
For parameters explanation, see the kernel documentation.
ip=127.0.0.1:::::lo:none --> Enable the loopback interface. ip=192.168.1.1:::::eth2:none --> Enable static eth2 interface. ip=:::::eth0:dhcp --> Enable dhcp protocol for eth0 configuration.
If you have multiple network cards, this parameter can include the MAC address of the interface you are booting from. This is often useful as interface numbering may change, or in conjunction with pxelinux IPAPPEND 2 or IPAPPEND 3 option. If not given, will be used.
BOOTIF=01-A1-B2-C3-D4-E5-F6 # Note the prepended "01-" and capital letters.
If the parameter is NOT given on the command line, the default will be used.
Run for parameter explanation.
Using encrypted root
/usr as a separate partition
If you keep as a separate partition, you must adhere to the following requirements:
- Add the hook, mark with a
passnoof in to run the check on the partition at startup. While recommended for everyone, it is mandatory if you want your partition to be fsck'ed at boot-up. Without this hook, will never be fsck'd.
- If not using the systemd hook, add the hook. This will mount the partition after root is mounted.
Extracting the image
If you are curious about what is inside the initramfs image, you can extract it and poke at the files inside of it.
The initramfs image is an SVR4 CPIO archive, generated via the
find and commands, optionally compressed with a compression scheme understood by the kernel. For more information on the compression schemes, see #COMPRESSION.
mkinitcpio includes a utility called which will list and/or extract the contents of initramfs images.
You can list the files in the image with:
# lsinitcpio /boot/initramfs-linux.img
And to extract them all in the current directory:
# lsinitcpio -x /boot/initramfs-linux.img
You can also get a more human-friendly listing of the important parts in the image:
# lsinitcpio -a /boot/initramfs-linux.img
Recompressing a modified extracted image
Invoke the function of the script with parameters
build_image outfile compression
It can be done by creating a new script with the contents of the function and running it with the above parameters. This will compress the contents present in the current directory in a file named .
"/dev must be mounted" when it already is
The test used by mkinitcpio to determine if is mounted is to see if
/dev/fd/ is there. If everything else looks fine, it can be "created" manually by:
# ln -s /proc/self/fd /dev/
(Obviously, must be mounted as well. mkinitcpio requires that anyway, and that is the next thing it will check.)
Possibly missing firmware for module XXXX
When initramfs are being rebuilt after a kernel update, you might get warnings:
==> WARNING: Possibly missing firmware for module: module_name
If these messages appear when generating a default initramfs image, then, as the warning says, installing additional firmware may be required. Most common firmware files can be acquired by installing the package. For other packages providing firmware see the table below or try searching for the module name in the official repositories or AUR.
Otherwise, if the messages only appear when generating the fallback initramfs image you have the following options:
- You can safely ignore the warnings, if you know that you do not use the affected hardware.
- If you want to suppress the warnings, you can install the missing firmware. The meta-package contains most optional firmwares. Alternatively, manually install the needed packages:
Module Package aic94xx aic94xx-firmwareAUR bfa bnx2x liquidio mlxsw_spectrum nfp qat_4xxx Firmware is not yet available. qed qla1280 qla2xxx wd719x xhci_pci upd72020x-fwAUR
- If you want to get rid of the warnings, but do not want to waste your system space on unneeded firmware packages, you can disable fallback initramfs generation altogether:
- Change line to in all .preset files in
- Remove fallback initramfs images: .
- Regenerate your bootloader config (e.g. for GRUB: ).
- Change line to in all .preset files in
No PS/2 controller found
On some motherboards (mostly ancient ones, but also a few new ones), the i8042 controller cannot be automatically detected. It is rare, but some people will surely be without keyboard. You can detect this situation in advance. If you have a PS/2 port and get message, add atkbd to the array.
Standard rescue procedures
With an improper initial ram-disk a system often is unbootable. So follow a system rescue procedure like below:
Boot succeeds on one machine and fails on another
mkinitcpio's hook filters unneeded kernel modules in the primary initramfs scanning
/sys and the modules loaded at the time it is run. If you transfer your directory to another machine and the boot sequence fails during early userspace, it may be because the new hardware is not detected due to missing kernel modules. Note that USB 2.0 and 3.0 need different kernel modules.
To fix, first try choosing the fallback image from your bootloader, as it is not filtered by . Once booted, run mkinitcpio on the new machine to rebuild the primary image with the correct modules. If the fallback image fails, try booting into an Arch Linux live CD/USB, chroot into the installation, and run mkinitcpio on the new machine. As a last resort, try manually adding modules to the initramfs.