initramfs

• initrd - old, pre-RHEL7, block based disk image
• initramfs - new, CPIO archive
• initrd vs initramfs - no fixed size, faster, easier to modify, mondern kernels expect it

lsinitramfs /boot/initrd.img-$(uname -r)      # view initramfs contents

Debian / Ubuntu:

sudo update-initramfs -u                      # update current kernel's initramfs
sudo update-initramfs -k all -u               # update for all kernels

RHEL / Fedora:

sudo dracut --force                                             # for current kernel, force to overwrite existing image
sudo dracut --force /boot/initramfs-$(uname -r).img $(uname -r) # also for current kernel
sudo dracut --force --hostonly --add systemd                    # for systemd-boot

sudo dracut --force --no-hostonly /boot/initramfs-minimal.img                   # minimal, for any system
sudo dracut --force --hostonly /boot/initramfs-hostonly.img                     # just for this system
sudo dracut --force --add-drivers "nvme.ko xfs.ko" /boot/initramfs-custom.img   # add xfs and nvme modules

Arch:

sudo mkinitcpio -p linux
sudo mkinitcpio -g /boot/initramfs-linux.img

Modify

Extract:

mkdir ~/initramfs-extracted
cd ~/initramfs-extracted
sudo zcat /boot/initramfs-$(uname -r).img | cpio -idmv

Pack, update, verify:

find . | cpio -o -H newc | gzip > /boot/initramfs-custom.img

sudo update-grub

lsinitramfs /boot/initramfs-$(uname -r).img | grep xxxxx        # verify change

Why

Drivers for:

• RAID
• LVM
• Encrypted FS ( ex. LUKS )
• Network FS ( ex. NFS, iSCSI )
• NVMe or USB Drives !!!

Also, root FS device name may change.

Creating a universal initramfs for a bootable USB

Ubuntu / Debain Universal initramfs

sudo apt update
sudo apt install initramfs-tools busybox

Configure:

sudo nano /etc/initramfs-tools/initramfs.conf

MODULES=most
BUSYBOX=y
COMPRESS=xz  # Or gzip for better compatibility
BOOT=local
DEVICE=
NFSROOT=

Add modules:

echo "ahci" >> /etc/initramfs-tools/modules
echo "ext4" >> /etc/initramfs-tools/modules
echo "vfat" >> /etc/initramfs-tools/modules
echo "usb_storage" >> /etc/initramfs-tools/modules

sudo update-initramfs -c -k all
cp /boot/initrd.img-$(uname -r) /mnt/usb/initrd.img

RHEL / Fedora Universal initramfs

sudo dnf install dracut

sudo nano /etc/dracut.conf

add_dracutmodules+=" base udev kernel-modules "
hostonly=no
compress="xz"

sudo dracut --force --add-drivers "ahci ext4 vfat usb_storage"

sudo dracut --force --no-hostonly /mnt/usb/initrd.img

Old Stuff

Manually create initrd ( obsolete ):

mkinitrd /boot/initrd-$(uname -r).img $(uname -r)  # SUSE-based systems