How do I create smaller initramfs images on Ubuntu?
If you are running Ubuntu, your initramfs images may be quite large. On my system, for instance, each initramfs took up about 100 MiB of space. Because I did not pay enough attention when setting up the computer I ended up with a very small boot partition, which prompted me to look for a way to make the initramfs images generated by update-initramfs smaller.
Caution: Any of the below may make your system unbootable. Please only copy the steps if you understand what they do.
Step 1. Fewer kernel modules.
Create a file called, say, /etc/initramfs-tools/conf.d/zzz-custom (the exact name of the file does not matter much) and fill it with the following:
MODULES=dep
This causes mkinitramfs to guess the set of kernel modules required to boot your system based on what is currently loaded and what hardware is present instead of indiscriminately including whatever could be useful to make a computer boot.
This saved me about 50 MiB, reducing the size of the initramfs from 100 MiB to 50 MiB.
Step 2. No GPU.
Assuming you do not need to interact with the initramfs (to debug boot problems or to type in a disk encryption password, say), you can disable the scripts that deal with setting up a graphics frame buffer. Doing so gets rid of GPU firmware, which at least for the amdgpu driver is a pretty sizable amount of data.
Adding the following to /etc/initramfs-tools/conf.d/zzz-custom may or may not be good enough:
FRAMEBUFFER=n
In my case it was not good enough. Since I boot from ZFS, the /usr/share/initramfs-tools/conf-hooks.d/zfs hook was active, forcing FRAMEBUFFER to y regardless of what /etc/initramfs-tools/conf.d says.
But of course you can add your own configuration hook to /usr/share/initramfs-tools/conf-hooks.d. You just have to ensure it runs after the zfs one by giving it a lexicographically higher name. So create a file called /usr/share/initramfs-tools/conf-hooks.d/zzz-custom and fill it with the same content as above:
FRAMEBUFFER=n
As long as you do not need a disk encryption passphrase prompt in the initramfs, this should not break anything. If you do, it is probably a bad idea.
This saved me another 30 MiB, reducing the size of the initramfs from 50 MiB to 20 MiB.